My opinion would be as such:
Muscle car-2 door intermediate rwd,big block V8 with minimum 300 horse(factory),true dual exhaust.IE-GTO,Chevelle,GTX,442,S/S Dodge,etc.
Pony car-2 door rwd,small block V8 with minimum 265hp(factory),true dual exhaust.IE-Mustang,Camaro,Javelin.
The general concensus is 1964-1974,but it may vary slightly.After the SD455 Trans Am,there were no rip snortingly powerful cars.Emissions and low compression smothered the V8.A 195hp 454 is nothing but a boat anchor.
A 1981 Z/28 isn't really a muscle car.It doesn't have the hp.
G body cars are not muscle cars.No hp.They all ran single exhaust.Crossover pipe,single to the cat,then split in two.Not real dual exhaust.
As usual,there is an exception,sort of.The GN & GNx.
A sports car by definition is a two seater.Corvette,Viper.Both over 500hp!!
Can't wait to see the new Camaro and Challenger!!
Anyhow,the above is my opinion.
For what it's worth.