The Gear Vendors overdrive and the ability to split gears is probably the best performing package unless you got yourself a G56 6-speed cheap (ha!) and could adapt it economically. Most anything other than the Gear Vendors will not bolt to the /6, for transmissions with an integral bellhousing, or to the bellhousing in the case of a transmission with a bolt-on bellhousing.
The only thing that will bolt in place of the NP435 and is closer ratio is the NP4530 which Dodge calls the NP445. The NP4530 is fully syncronized and had 4.56, 2.28, 1.31, 1:1 ratios while the NP435 is usually 6.68, 3.34, 1.66, 1:1. There may be 4.78, 2.39, 1.37, 1:1, but that one appears to have been used in earlier trucks before the NP4530/445 was introduced. I consider the NP4530 to be the best 4-speed manual transmission for highway towing. It can be identified by the blue tag with ratio 4.78.
The NV4500 is geared much like the NP435, but adds an overdrive. It's still a wide-ratio transmission. For a while I drove a 1996 Dodge 3500 4x4 with a V10 and the NV4500. It was fine until I used it to pull a heavy load in the hills. Loaded I wanted the gears closer together even with 300hp and 450 ft/lbs, but the truck weighed about 8,000 lbs and I was pulling about that much more. Oh, it got 9.5 MPG combined city/highway not towing.
All this said you are still at a massive power deficit compared to a modern truck. It's faster, cheaper and easier to buy a more modern truck for $4000 than to install overdrive in yours. When I needed to tow more I got rid of my 225 powered Club Cab D100 and bought a 1972 B300 van with a 318. I'd do it again without hesitation.
I... don't... understand.......
Although I plan to eventually put a turbo Slant into that if I get around to it or this engine quits. His truck came with a Slant.