If a six came in the truck it should have a 3.73 if it had a manual trans.if it was a auto it could be a 4.10 you can change the tire size up down to get less are more gear also. if running the 400SB I would leave the rear alone and go with a 700R4 auto overdrive. 64 to 72 rears will fit but they are a inch wider just check clearence with tire if you change. also 71 and 72 will have five lugs and some of the 70 models did also.
