You all might consider that only the first couple of years of Dodge NV4500, I think, it's Dodge, had a compound low that I believe you'll want for crawling around in RV parks or other tight spaces when loaded. The rock crawler guys like the SM420 for that reason. SM465s have a good gear spread too but the front bearing retainer is too big for the sm420 bell housing.
The combination of an SM420 or 465 and a Spicer auxiliary either a 3-speed or a progressive 4-speed with give you good gear splits (~300 RPM) and a 17% overdrive. Grigg has such a setup as do I. Ernie, who advertises in the ATHS magazine has a wrecking yard in Riverside CA and when last I talked to him had at least one of these.
The common Spicer model is a 6231A or a 6241A or a 5831A. You want the "A" because the others have a deep under that was used with a five speed that didn't have a compound low. These were used in all sorts of heavy trucks like tandem-axle dump trucks where 2-speed axles either weren't available or were prohibitively expensive. Gas powered or 4-71 GMC diesel tandem-axle truck tractors also used them. The next larger 8xxx series is total overkill and was used behind like a 250, 335 or 350 Hp Cummins, (855 cu. In.) or a Hall Scott 1090 cu in.
Once properly set up, this combo will be reliable as a stone axe but the overdrive on the brownie will have a whine. I will take some practice to get on the the split-shift but in time it becomes second nature.
