The transmission gurus will know better than me, but I believe the input shafts are different sizes as well as lighter duty planetary gear sets. I'm also inclined to think the governor assembly in the overdrive section would be different (assuming the gas engine shift points occur at higher engine speeds).
Off the top of my head, different bellhousing/bolt pattern, likely lighter duty (4 pinion) planetary gearsets, lower clutch counts in the various clutch assy`s and the governor weights are lighter.