As noted before, it is going to be impossible to turn that C20 into a K20. The 4 wheel drives use a leaf spring suspension, while the 2 wheel drives use coil springs. Just having to swap out the entire spring setup would make it prohibitive. Sure, technically it could probably be done, but the cost would be so high that it wouldn't be worth it, the body swap would be far cheaper.
'66's can be found in 4 wheel drive. Take a look around a bit and you can probably find one. If you are set on doing the body swap, then that is your choice. It is, after all, your truck to do with as you please.
Now, I have in the past test fitted a '63 body onto a '77 3/4 ton 4x4 chassis. The cab and bed fit on pretty nice, but you do have to do some fabrication to mount everything. It's not like the newer body mounts will work, so be prepared to fabricate mounts or at the very least modify existing mounts to make it work. It isn't all that hard, just very time consuming.
