My first project I placed a firebird rear in my 1950 1/2 ton with 3.41 gears along with a 350:400 combo.During final frame mockup (I never finished this one...:( See the gallery!) it fit quite well. However with 235 tires on the back it was so close to rubbing the inner bed side. I can't remember what its width was. I believe it was about a 78 firebird. Somewhere in there. Crazy how fast 15 years can go, eh?!
I would definitely measure both back to back and front to front and also have an idea of what size tires and wheels you want to run.
It is frustrating doing this as so many factors come into play. I wanted the same thing (still do some times)... I just need someone to tell me what to do! Good luck Spotbiltxo!