Ok we'll make this fair and start with an 84 Mustang. Now of course they came in more then one engine selection, but i'm just going to put up the 302ci (5.0) types for that year. Here we go:
1984 - 165hp
1985 - 210hp (0-60 in 7.1 seconds, 1/4 mile in 15.5 seconds @ 89.7 mph.)
1986 - 210hp
1987 - 225hp (0-60 in 6.7 seconds)
1988 - 225hp
So you can clearly see that these Mustangs weren't that much better performance wise. And they weighed like half the weight and had more HP. Pathetic. But lets go just a little farther....
1989 - 225hp
1990 - 225hp
1991 - 225hp
1992 - 225hp
1993 - 200hp - (Cobra - 235hp)
I'll stop there. I think i've made my point. Don't get me wrong, some Mustangs are really nice and i'm not trying to bad mouth them at all, i'm just saying, power for power and pound for pound the Montes would eat the 5.0 Mustangs. Although anything after like 1993 or so the Mustangs would start to kill the SS's. Around like 96 I think they started to hit like 305hp+ or so. (Might have been 95)
I say, if you want an older "muscle" car and you have the money, get an 87 Buick Grand National GNX. Turbo charged @ 275hp stock. 0-60 in 5.5 seconds. I forgot what it can do for a 1/4 mile time....but it hits about 97.7mph at the finish line. Only an 87 though. Grand Nationals were made from 78-87 but the 87 GNX was the most powerful one, not to mention, last one ever made.