It sounds like you have a front-motor, rear-drive arrangement. I have taken an interest in the front-motor, front-drive '73-'78 GMC Motorhome and have begun researching it. I ran across this link to Applied GMC that lists 3.07, 3.21, 3.55, 3.70 and 4.10 gearsets. These are to fit the TH425 Toronado transaxle, but will show that these are the most popular sets. I'm pretty sure the Motorhome came from the GM Pontiac plant originally with 3.42 gears, so if you have a good, strong motor, 3.42 might be the best all-around ratio to use.
http://www.appliedgmc.com/level.itml/icOid/482

I'm pretty sure I'm gonna buy one of these classic motorhomes and do a little fixin' up. Maybe get Pat to put a rotund rat motor together for me.