Well, judging by your user name I'm gonna guess you have a 73 and can get an 89 frame.
If that's the case you'll need to build a midship crossmember and body mounts. You might run into some issues at the core support too. I have heard the height of the mounts is a bit different.
What exactly is wrong with the frame on your 73? It's usually the body that goes but the frames survive...
Rene