I'd have the rear of the frame and body (upper wheelwells) seriously examined..

A truck that old just has to have some hidden delights underneath. <img src="/forums/images/graemlins/frown.gif" alt="" />

Are you set on putting it in a SWB? There's plenty of LWB trucks around, including a 91 that was here locally that are probably in better shame than that truck is underneath.

1999 Mitsu Montero - Crappy Weather
1992 Isuzu Pickup - Zombie Apocalypse
2008 Saturn Sky Redline - Nice Weather