I read your pieces with great pleasure, and being European with lot's of new insight.
What i wondered was what kind of breaks where used? Drum-breaks? How did they tackle the power? Where there upgrades available at the time? Or was it no issue?
The same goes for suspension, i guess feathers where used or also springs? Where they adapted too? (a part6?)
Ok so let's look at some of your questions and I will try and answer them the best I can.
First, brakes! Most had front disc brake with rear drum brakes as standard equipment. However, some special models did have rear disc brakes as an option from the factory. Corvettes were known to have rear disc but so did some 69 Camaro's. Not very many did though. Today, you can buy all kinds of upgrade kits to convert to 4 wheel disc and have better braking than they did back then. Even the fronts will be better.
Power. There were some factory options you could add when ordering your car. Some were done so that they did not have to truly tell how much horsepower was being produced. Sometimes even the dealers were not totally sure for want I am to understand. But, when ordering these cars, most came with lots of horsepower to begin with.
As for upgrades, there were plenty if you had the time and money just like today. Putting bigger carbs and a highrise better flowing intake was the quick and easy way. Pulling the engine apart and putting in a bigger cam, bigger valves or sometimes even boring the cylinders to make larger displacement were all available. Today there are many more ways to take the same engines and really produce some big hp numbers. Ignition systems also had upgrades for better reliability, since most were points and condenser, not electronic ignition like cars for the early 70s. They also had no ECUs or catalytic converters which were not mandatory until 1973 I believe. This made changing the exhaust to true dual with headers possible and made that big classic muscle sound. Cats really quiet the exhaust note more than you might think. Fuel injection was more mechanical and rare, but was around. Having carburetors made for easier ways of obtaining better fuel flow and higher volume.
As for springs, I was trying to come up with a way to explain them but was not confident in a good explanation and therefor left it out. Many cars from the 60s had rear leaf springs, but by the early 70s were switching to rear coil springs. Leaf springs did stick around for quite awhile, as the Nova and some others like the Cuda continued to use them. They were easy to raise the rear ride height, which was the classic look, by adding one or two "leafs" or by adding a block between the springs and the rear axle tube. They did ride harder and made the suspension feel different is all I can say. Not much, but different.
If you want to see a classic muscle car look, take the 69 Camaro Race Car, lower the front suspension just a little and then raise the rear to the max. That is what was normally done, and putting wide tires on the rear as well. Not quite that wide, but you will get the idea.
Here is an image that might help give an idea of a basic suspension and brake system, not an older car but helps explain better I hope. Of course the front suspension is wrong, but does have leaf spring rear suspension.
Hope that helps to answer your questions.
, funny you should mention the leaf spring thing, I had just given my point of view to @Thorin Cain about how it may be programmed on the original suspension but as you put on the custom adjustable it turns into a coilover system, this is just a hunch, it's going to be hard to prove, but I think PD have put more in than what we've accounted them doing. There's also the MLSD and brakes that may have different versions that are programmed, just haven't had the time to finish those tests to see how it goes. If you have ideas about the way of going about testing these, I'm all ears and eyes 👍.
Just not on bias ply tires.
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, we have someone who's driven some of these 👍, and has experience in engines and trans 👍, if ever you get the fancy of sharing your IRL experience with a bunch of guys who are trying their best at doing this in GT6 please give me a holler 
