I have A 76 model 4wd 3/4 ton Chevrolet pick up and I want to move the fuel tank from out side the frame rails to between the frame, does anyone know of anything that will fit with minimal fabrication, I have the ability but dont have alot of time.  

Have you looked into manufactured fuel cells from Summit Racing or Jegs ?
You might also want to try marine supply stores. They can get different custom size aluminum tanks from their suppliers. The only issue is if you can get a 15 or 20 gal tank in there with minimal fab.
I've got a '77 1/2 ton 4X4 with the tank between the rails. I'm not familiar with the tank setup on a 3/4 ton, but are there any obstructions to where you couldn't get a replacement tank from a 1/2 ton in there ?
LMC truck carries these tanks, I belive. They can probably give you dimensions on it but they have no tech support there - LMCTruck.com  

i think a tank from a suburban or blazer could be refit into the chevy trucks with minimal work specially if you could get the forwrd crossmember from the blazer. Redbeast where did you find a 77 with inboard fuel tank that cant be stock? never seen a chevy from 77-87 with anything but outboard fuel tanks  

No matter what you do, that is a big project and surely will require time and fab.

Be sure and let us know what you end up doing though!