Failure around the steering box is not uncommon on GM trucks with that basic frame design right up through the 87's, and there is a kit I've seen to strengthen and repair the damage. You might check into that, as a repair is likely to be much less trouble than a frame swap, even to another '63 frame.
If the wheelbase is correct the '66 frames should be real close to the same, but I would not choose either of them unless you want a 3/4-ton or a 4x4.
'63 to '66 are very similar, but I believe there are some year to year changes. Wish I could detail them, but I haven't own one of those trucks since about 1980 and no longer remember many details.
