First gen RX7's had a pretty stout live rear axle, I had 5 of them. A red 79 with an automatic, forest green 82 with 5 speed, black 4 speed 83, a charcoal 84, and another black 83. The engines are also very temperamental and costly to repair. They are a blast when they run right, but when they wear out there isn't much you can do with them. He probably dropped a mustang 5.0 and 5 speed in with aftermarket motor mounts. Pretty simple swap, and there is a ton of speed shit for mustangs. The engines are also pretty simple to fix.