Any time you change a factory engineered design you can get yourself in trouble. Automobiles have been designed from the ground up with one thing in mind: economy, but not economy such as gas mileage or price for the consumer. This economy is the per unit cost. Manufacturers have always wanted to economize on the building of their cars. This is a simple fact. To that end, every part of a car is balanced to perform with the other parts and is only built as heavily as it needs to be, rarely more and often less resulting in cars which are built flawed from an engineering standpoint right from the factory. Automotive engineers have been capable of building perfectly reliable cars for decades, but not with the constraints given them by the accountants in charge of production costs and boards of directors interested in quarterly profit.
So, along comes you, with great ideas on how to make your favorite body style into a boulevard cruiser with horsepower galore and modern handling. Now the right way to do this is to build the car from the front to the back, bottom to top, using common sense and balanced parts which you know will work with one another. Unfortunately this is often beyond the means of many rodders so what happens is things get done in little bits and pieces and this can lead to a lot of troubles.
When you make modifications to a car, you have to think the whole thing through. When you channel a whole lot more horsepower through a car than it started out with or was designed for something is going to break. Take a drive shaft designed for a motor which doesn't exceed one hundred horse. If you use it to transfer 300 horse, it is not going to take it for long. Either you are going to break universals or you will actually break the shaft itself. I had a drive shaft at one time that came off a car which was overengined. I wish I had kept it because it illustrated what a powerful motor can do. This shaft was actually corkscrewed into a sharp point and almost looked like it was made that way.
Rear end gears also take a beating and can fail. A lot of people think that because a rear end is massive in appearance it will take lots of horse. Often these old rear ends are massive simply because the manufacturers, lacking high quality steels and the kind of metallurgy we have the benefit of, built the parts massively in order to assure that they would not break under those contemporary loads. Increasing the loads with a modern engine will cause certain problems.
The same can be said for old suspensions. If you are going to dramatically increase the horsepower flowing down your driveline, make sure your suspension is going to be able to take the added torque and twisting. Just adding modern shocks is not the answer. Although they serve a valuable purpose in dampening road shock and bounce, a new set of flashy shocks will not make a forty year old set of leaf springs happy about absorbing the forces imposed on them by stomping your foot down on the accelerator when the light changes to green. You will often see old cars actually twist sideways of the start and dart about until they reach a speed where the engine is actually pushing the car at the same speed as it is moving. It never fails to horrify me that on lookers ohh and ahh at this as it is certainly not either safe or desirable in a straight line car. It is a calamitous handling problem brought about by a body and chassis which are trying to absorb loads that they were never intended to encounter. This can end up with everything from frame cracking, spring breakage and other failures through to a complete loss of control of the car.
Another problem is that of under building an engine itself. The first and most often made mistake is over carburation. If you put a carb on that pumps more fuel, it will not make the car go faster, it just makes it run richer which is hard on the engine as a whole. If you are going to put a high performance part of any sort on a motor it has to be done in concert with others. No point having a high performance coil if the original coil sparked all the plugs and all the gas exploded.
When you are building an engine, start from the inside and work out. Have the crankshaft machined properly, add rods and pistons designed for more efficiency, put in that lumpy cam but make sure that the rockers and valve line can support what the cam is doing. Put on headers so that exhaust gases can get out of the engine more smoothly and a performance intake to match the carb and insure that the air fuel mixture is drawn in efficiently.
Research all the bits to make sure each one works well with the other. Not all performance parts work well with each other so it is best to look into it, and not just with the local garage. There are an awful lot of so-called experts out there who think they know or make up the answers to questions but the money isn't coming out of their pocket book. Find a reliable and knowledgeable performance shop, get their advice and balance that with information available from the manufacturers themselves. Have a look on the Internet. It is amazing how much reliable information can be had there.
Once you have an engine that is built well, marry it to a transmission capable of taking the added horsepower. A good shift kit on an automatic can do wonders and nickel gears in a standard is often a good idea.
If you are not changing certain things on a driveline, make sure that they are going to be capable of holding up to the new power. Don't skimp when it comes to safety related items. It is better to go without that chrome alternator than to ignore the front springs and the added loads you are expecting them to cope with.
Some things are very simply and inexpensively remedied. If you are running a lot of power through the car and have added weight to its front end with a larger engine and heavier transmission then you might be light on the backend. I remember as a kid seeing the street draggers ( I might have had the odd drag myself) with lead poured into the trunk pans of their cars to improve their rear end hop and traction. Something similar can be of help to you as well. Remember that you need to find the center of balance for your car and try to keep it where the designers put it. It may require weighting the back a little but that will result in a car which handles far more safely and with predictable habits. Too much weight forward and you can have severe handling problems especially during an emergency. ( Have you ever tried to take a 429 Mustang around a sharp corner under acceleration? If you have you will know what I mean.)
There is one more thing that you should think about when it comes to Rodding. That is your legal liability. If you build something and then take it out and kill somebody because of a poorly thought out idea, you could end up in some very serious trouble. There are both criminal and civil laws to consider when you are doing this kind of thing. If a wheel drops off your car and a Ministry of Transportation inspection determines that you have done something stupid then it is possible that you could be charged with negligence or worse. A good civil lawyer could also make your life miserable for years if your hot rod or custom is suspected of causing an accident, especially one in which a life is lost.
This is something which you have to guard against for your own sake as well as for the sake of the hobby.
I enjoy Hot Rods and Customs, and I certainly wouldn't want to see them legislated off the road like they have been in Europe, but if that is not to happen then you need to be responsible and build safe and sensible cars. When you build a car have it checked for safety by a professional and don't skimp when it comes to safety items. A nice big shiny motor is fine but if it is laying in a ditch in a twisted wreck, it loses some of its appeal.