All of the nv4500s had a granny gear first. All of the Dodge nv4500's had a 5.61 1st gear. Chevy nv4500's had a 6.34 1st gear for '93 and '94, and then they went to the same gearing as the dodge transmissions.
Rock crawlers like the sm420 because of the small size, light weight, and the 7.05 first gear which is the deepest you'll find in a light duty transmission. The sm465 will give you a 6.55 1st gear. All of these granny gears are nice to start a heavy load off the line or to idle around parking lots; however, the main gripe that serious haulers have with these transmissions is the 3-4 split.
The sm420 and sm465 both have 1.7 3rd gears, and the nv4500 has a 1.67 3rd gear. It's a big jump when pulling a lot of weight to go from 1.7:1 or 1.67:1(3rd) to 1:1(4th). That's why the 6 speeds are sought out by haulers, and why you see the medium duty transmissions having a 1.2:1-1.4:1 gear before the direct 1:1. If you've hauled any weight with one of these 4 speeds or the nv4500 with a gas motor, you know about that 3-4 gear split.
Brownie boxes are great if you can find one at a reasonable price, but more likely than not, it will be cheaper to use a ranger overdrive or an nv4500.
