starnest hit the nail right on the head -- the power drop for 1971 was TOTALLY the change in the way power was rated. it took another year or two before smog related items started taking a toll as well. If you check the engine specs in a Motors or Chilton's all make service manual you'll quickly see that just the power and torque numbers changed, everything else remained the same for 1970 and 71 (or was that 72?).
There are lots of interesting 70s cars. The Fairmont/Granade and even the late 70s LTD (NOT that huge LTD II!!) platforms are real easy to build a muscle car on. Light and will handle the power. V-8 Vegas and Pintos (and hopped up MIIs) were quite popular in the late 70s. What the factory didn't do was easy to do with mostly factory parts and a bigger, or just better, motor. The MII (and don't forget the Maverick Grabber!) original 302 wasn't much, but it didn't take a lot to give it a kick in the pants, even with junkyard older Ford parts.
The AMCs of the time are way under appreciated. MUCH easier to throw an AMC V-8 in a Gremlin or Hornet (since this is a 70s discussion I won't mention the late 60s Rambler Americans...) and have one heck of a hot little car. Gremlins came from the factory with 304s, but even a 232 six Gremlin with a stick was a lively ride -- no fours for AMC until 1977! AMC made 360 Hornets for a couple years, not just the 71 SC/360 -- you could get a 70 or 71 (and I think 72) Hornet Sportabout wagon or four door sedan with a 360/2V and auto trans. I think those Hornets look much better than the Vegas, Pintos, and Mustang IIs (Hornet hatchback and two door sedans... HB didn't come out until 73). They are much easier to stick a V-8 in (any small block will fit, BB would be tight though) than the other makes -- no special kit required since there was a factory V-8 option, and that wasn't stuffed in there as an afterthought either!