Why is my top speed 70 mph?

It could be a trans/converter and maybe a timing issue. It looks like you have a long list of possibilities but it will pay off when you find the problem and you will know your car inside and out. If it has a rear end cover it's most likely an 8 1/4 if it was originally a v8 car and a 7 1/4 id not. I doubt you would have high gears if you have either.