“...Why? They can’t hold a candle to today’s muscle cars....”
Because back in the day:
I paid 350 bucks for a 69 SS Chevelle; (in 1977)
325 bucks for my 1967 Mustang GT 390; (in 1979)
700 for my 1971 Mach One (in 1980)
250 bucks for my 1968 Fairlane (in 1984)
Today’s cars can’t hold a candle to those prices; and I could work on anyone of them, with aftermarket OR OEM parts that were cheap and easy to install.
Now? Not so much
The years you mentioned were NOT the muscle car days.