Swapping to a TH350 or 400 wil not gain anything over the 4 speed, high gear will still be 1:1 in either case. The TH700R4 with the overdrive or the gear swap will make a difference. '63-'70 will be a more or less bolt in swap. I do beleive that there is a slight difference in the arms and Panhard bar attachment, but nothing that can't be easily taken care of. You probably have the 4.10 gear, optional in the 1/2 ton, along with 3.08. The 3.73 was the std gear. Aftermarket gears are redily available, but you do need to confirm which carrier (the part the ring gear bolts to) you have to determine what ratios are availble to use without changine the carrier out.
