Re: 400 small block
Putting the S10 body onto the K5 frame is the easiest solution! The S10s frame is so bastardized because of the IFS that it requires TONS of new or modified crosssmembers, frame horns, mounts, etc! Alls you need for the body swap is your body mounts from the S10s frame, then mark & weld to the K5s. The title should be easier since its still a K5 driveline. Rust free K5 bodies are rare as hens teeth here in the rustbelt, but Ive seen truck cabs on them before with a 4.5' bed. If she'll run, do a sawzall job to the fenders, get some 35 or 36s and go to town! Also loose the top and doors for a MAJOR weight deduction!
As a rule, stock K5 ball-joints dont like any tires over 12.50 wide, or 38" tall. Sounds like an excellent project, and fairly less complicated than an SFA swap on an S10! Good Luck.
1989 GMC S-15 4x4 4.3L/700R-4, 3.08s, 31" Sport Kings For Sale
1985 S-10 4x4 Soon to have 3504BM/TH350/D44/14FF, 35" Boggers