I'd go with a set of 4:11 gears from a late 60s truck. Overdrive trans would be pretty pricy plus you'de need a really heavy duty unit in that bus. You could ad a power brake unit to your existing brakes for better stopping. Keep it simple and don't try to set any speed records with that big truck.

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed