NP435 Trans Swap Help

A 518/46RH will work, but you’ll have to add a lockup and overdrive off switch. It’s best to get it from a donor truck so you can grab the drivelines, too. You would probably be getting into the spline count complication, so you’d need to grab the matching 205 transfer case, but an NP 445/435A or 435D according to Novak The Novak Guide to the New Process 435 Transmission
(known as the close ratio, the 435 is known as the geometric transmission) is a lot fun as they have about one third of the shifter travel as a geometric geared transmission and can be shifted quickly. But, they’re almost always in full ton application and getting hard to find. You’d have to change or modify drivelines or use 1310 to 1350 U joints. And if you’re in the 33 inch or taller tire range, the whole deal works a whole lot better with 4.56 and lower gears. The dually trucks I’ve seen outfitted with these transmissions came with 4.56 or 4.88 gears, but 5.13 or even lower would not surprise me for heavy duty utility service. The best option may be a Dodge NV4500 5 speed (I’ve even seen a very few of them in a half tons!), but they’re usually pretty pricey. Speaking of pricey, something for your viewing entertainment. But it talks about the different spline arrangements and strengths of different units.
NP203/NP205 Doubler Kit
I’d love to outfit a truck with one of these setups. Being able to use front axle drive would have been handy off road more than once when the rear started trying to slide out off camber embankments…