back when i had my 85 nissan there were a few writeups online of the 4.3 swap, and i think a company that made the mounts for it, but i never got around to it, and that was like 7 yrs ago..
Try 4x4parts.com They're a site dedicated to Nissans and have some write-ups on doing engine swaps, solid axle swaps, etc. Most that I've researched are w/ a 350, but they might have just what you're looking for