The transmission ratios are going to be the same, the differential ratios will depend partially on what transmission is in it, and partially on what option ratio was optioned for it. The engine option really does not come into play. 3.90 was the standard gear, with 3.70 as an economy option. 4.10 was generally used in trucks with overdrive, and the 3.38 was the Hydramatic differential ratio. There was also a 3.92 option, that one is for the rare Positraction axle, which is a Spicer 45. Note; The Spicer was the standard axle in the GMC, using either a 3.08 or 3.92 gear. The code for the axle will be stamped on the carrier face, on the passenger side of the carrier.
Personally, I would go with an overdrive transmission over swapping the rear end gear. Keep the lower gearing for in town use, but still have a good highway gear using the overdrive.
