Kieth any of the other two would be a viable option for an overdrive, the 3500 actually was replaced with the NV-3550 it only has a torgue rating of 300 ft lbs, same as the t-5 trans, the NV-4500 is a very tough trans, also very heavy trans, at 200 lbs, if you decide to get either of these two trans, get one from the GM line up as the input shafts are differant from the mopar version, also one not thought of by many is the Asin-ax-15, used in the jeep from 92 to 99 5-speed, though the MFG never gave the torque ratings the jeep owners that have them say they believe its in the 375 to 400 ft lb ratings these trans were used in both four wheel and two wheel applications if found in a two wheel model you wouldn`t have to find a tailshaft housing for it, all these trans can be installed in our trucks in one of two ways, either by adapter to chevy bellhousing, or by specific bellhousing sold by Novak or thru Summit, and or Advance adpters. Novak website list all the trans specs and adapters/bellhousings....happy reading...
http://www.novak-adapt.com/catalog/a...s_adapters.htm