1958 GMC 336 tranny match up

[1project2many]

If you're OK with keeping a 4 speed the M420 is a tough transmission. The original NAPCO conversion was an add-on kit and used a divorced transfer case. Divorced transfer cases were common on trucks in the '60s and '70s and one advantage to using this style case is that you don't have to change any of the original engine or transmission configuration.

A later SM465 may bolt to the original bell housing. It does in the Chevrolets but I'm not going to promise it will work in the GMC with the Poncho engine. If you can use an SM465 then you can use a later transmission/transfer case assembly.

Many later 4X4 trucks have a wider stance which comes with with wider axles and wider spring mounts. Finding axles that fit under a TF with no modification can be a challenge. IIRC the 72-ish Dodge 1/2 ton trucks are very close. They also happen to use a divorced transfer case, the NP205, which is very tough. International and Ford trucks from the early '70s also used a divorced case but the axles may require more work to install.