I wouldnt mix the 3.91s with the 3.09 1st unless I had pretty tall tires . You are getting close to grany gear ratios with 14" tires. That would get old quick ! 3.55s would be the sweet spot .
Actullay I had an 82 D150 with a 318 and 833 OD combo that I installed an 8 3/4 in from a friends old 73 B100 van.
Turned out it had 3.91s in the back ...lol Worked great but the truck had 30" tall tires.