I think the NV4500 goes all the way to 02. Ours is a 98. Not sure the frame needs hacked on those, as they have a pretty deep area, the major headache on 2wd trucks from the 70's is that the steering runs behind the crossmember, while the 4wd does have to get the frame modified if I recall. You also can get into the firewall pretty easy in the 70's trucks.

 

6 speed was optional from 99-03. I have seen a few new body style 03's with the common rail and 5speed. Ultra rare. But for ease of conversation explaining it to a dodge newbie. I didn't want to get down into the nitty gritty. They also used light duty versions of the nv4500 behind Chevrolet 454's but a cummins will destroy the light duty versions. And also when u get one made to bolt to a cummins it is much easier to install. Although the Ford 4 speed bw t19 will bolt to the cummins bell housing and be plenty strong, it is practically useless without overdrive

 

I agree completely about needing the overdrive by all means, just the NV5600 6 speed actually was harder to find than the NV 4500. Much fewer NV 5600's available in general. I also didn't want the range of useable transmissions to be so narrow. Lots of good info here: High Impact - NV4500 5spd