I think you'll have trouble with the fit, a buddy of mine did this with his 72, they put it on a 77 chassis. I can't remember all the details, but they had to do alot of work to get the body lines to match between the cab and bed. My vote would be to put the later axles on the original frame. You really may want to consider using a divorced T-case though, you could keep your original tranny if you wanted and there'd be no need to modify the trans tunnel. Good luck with it and keep us posted. This is a swap I'd like to do some day if I run across a 64-66 body.
