Just so we're clear, we're looking for the fastest car in stock form with no mods, American, and pre 75?
70 Chevelle SS LS6 if optioned correctly. The M21 was a little faster than the TH400, and the 4.10s were faster than the 3.73 or 3.42 cars. There were a very small handful of COPO versions of those cars that came with no radio, no A/C, no power steering, and no power brakes to shave every possible pound.
They are also incredibly rare and sell for well into the 6 or even 7 figures in some markets. One report I heard was that they made about 300 of them, only about 155 of them got the 427, and only about 24 of them had the LS6. Total COPO Chevelles known to be still in circulation is estimated at 66.
You'll find that the fastest ones are the ones that get pushed onto the paddock at a Mecum auction and really rich people buy them. I, for one, would rather buy an I-6 Chevelle for $3000 and drop in a junkyard 454 with modern heads and a nice cam. Blow away all the LS6 and COPO cars for $6000 instead of $6,000,000
The real allure of buying a car from the muscle car era was not necessarily in buying the fastest stocker, it's that you could buy a bone-stock base model for way cheaper realizing that the only real difference between the $500 base model and the $50,000 rarity is that one had a big engine and the other one had a small engine. So, if you're buying a muscle car to be fast in stock form, be prepared to pay insane amounts of money. If you want to get an even faster car for 1/10th the investment, it won't be an original stock car. This is exactly why I got a 67 LeMans instead of a GTO. If I paid big bucks for a wasted GTO, my best investment is to restore it to original. Then I have a 360-gross-hp car (maybe 275hp by today's standards) that needs race fuel and an insurance policy bigger than the one on my house. Instead, I will have about $20k total investment in a LeMans with a 550hp LS that drinks pump gas or E85.
Fastest stock muscle car is bloody expensive. A fastER non-stock muscle car can be dirt cheap.
Edit... having said that, I am not in any way poo-pooing your desire to own a piece of American History. We NEED people like that who are interested in keeping those rare gems alive, I'm just offering my opinion.
If you are willing to go not-stock, then you have really opened up the can of cheap worms. By 1972-73, smog laws and new SAE hp rating criteria caused hp numbers to drop like crazy. One year you had a solid-cammed, high compression whopper and the next year you had a tiny cam, low compression wheezer that (as if it's not bad enough) the numbers on paper dropped because of the new rating methods. A 1970 Chevelle SS might sell for $10k as a complete basket case, but a 1973 Chevelle might sell for $5k as a runner. Perception and market demand. For many years I was the king of 70s boats and sleds. Buy wheezer 1973-ish car (knowing that it was the same basic guts as a 1969 of the same name) and build some big-cube engine for it.