Sounds like there isn't much hope in it running. Would swapping the body over to a running truck/van be viable, or would taking the drive train out of the donor and putting it into this make more sense?
Swapping just the body to a modern chassis would be awesome - better brakes, frame, steering, everything. A drivetrain-only swap would maybe be easier, but not net you as much in the end IMO.
x2, but it's a HUGE project. A 2WD Chevy S10 is a super common frame swap for all kinds of different vehicles and hot rods. You'd need to have the frame and driveshaft shortened to match the wheelbase.
Use a late 90's 4 cyl for simplicity and low cost. The thing is, I have no idea how close the body mounts and frame locations look between the 2.
You also might have to get tricky with the location of the radiator and engine, due to the vans body.
It would be easier to swap something like a simple, carbed 4 cyl engine like a Ford 2.3 and a 5 speed transmission. You'd still have to deal with the original frame, brakes and suspension, etc.
I wonder how close that would be to an Astro Van wheelbase?
They're subframe vehicles, unfortunately.