You are going to be fabricating any way you approach this.
Since you are converting to four-wheel-drive you are already dealing with the track width issues of any newer frame and front suspension, and may also be narrowing the front axle.
Ignoring those issues for the moment, either you are fabricating suspension and drivetrain mounting, or you are fabricating cab and sheetmetal mounting. Either way you are biting off a pretty big project.
I've seen a 1-ton GMC of this era with a Cummins under the hood and know of others. I'd suggest you investigate the dimensions of the 3600, 3800, and 4100 frames. Each is incrementally a little bigger and thicker and the longest of those is 137 inches wheelbase. The 4400 and 6400 have 164 inch wheelbase. Longer chassis were used under school buses.
Compare those dimensions to the modern Diesel powered pickups built for use similar to what you plan and see if that tells you if you need to swap to something newer, or just a bigger era correct truck.
I'm not advocating either approach, but rather suggesting the decision be made based on a solid understanding of the options and implications.
Personally I'm working on a number of projects including one that is a frame swap. I've revisited the logic that lead me to the swap several times and keep coming back to it being the right choice to meet my objectives on that truck.
Frame and suspension modifications are hard enough to pull off you want to be sure you know why you're doing it before you start.
