You could use a 67-72 frame,i think they may even be the same or close to the original one..you could use a 73-87 frame too,but it will require more work relocating cab mounts ,which may have to be done to the 67-72 also..
I would say its easier than trying to make the 2wd frame in it now into a 4wd,that would requre a lot of rivet removal and bracket swaps..too bad you cant find a 66 4x4 frame in a boneyard--then it would be a lot easier--and it would have a divorced rockwell transfer case,which allows you to put just about any tranny in front of it,they use a short driveshaft to couple the two up..
They did put 3 speed muncies in a few 67-69 GM trucks but I only saw two of them in my life--one in a 69 K5 with a dana 20 transfer case with a straight six,and the same setup in a 68 GMC 1/2 ton 4x4 with a 307 V8 --it would be tough to find one today if even possible they were so rare..