Hi Ron,
I don't know what exact rear axle to look for, but I would start by looking for just the center section with faster gears, keep your housing under the truck. Call a bunch of old truck junk yards, someone will either have one, or know enough to tell you you're on a wild goose chase.
calculate speeds here:
http://www.onlineconversion.com/bigger_tires.htmI like the last on the page, and just guessing at your info looks like a 5.something rear axle would solve the problem.
Then if you want an OD to either go with or in place of an axle swap I would consider these models:
Spicer 3053A 5 speed with most commonly a 0.85 OD ratio as found behind many Multi-fuel engines in Deuce and a half military trucks. This is probably the cheapest and easy to find OD 5 speed, and plenty strong. It's overdrive, but not a very aggressive one. They come with an SAE #3 clutch housing, and a #3 to #2 adapter ring as well.
Fuller FSO-6406a and a FSO-8406a, both 6 speed overdrives with 0.78 ratio, but perhaps still on the large side, about 359 lb dry, they are single countershaft transmissions. Can be found in some medium duty trucks, reasonably new ones. These are pretty expensive and difficult to find used, but every now and then I hear about someone buying one for next to nothing, so worth looking for. Usually have SAE#2 housings, not sure if #3 was an option?
If you want more gears, and don't care for a two speed rear then think about a little Roadranger, both 9, 10, and 13 speeds, in overdrive and direct drive, probably in about this order of desirability:
RTO-6610
RTO-610
RTO-6613
RTO-613
RT-6610
RT-6609A
RT-610
RT-8609
RT-6613
The RTO stands for overdrive, and ratio on these models is 0.80.
The 9 and 10 speeds weigh about 425 lb installed and with oil, add 200 lb for the 13 speed (13 speed probably not a good option for your truck). Come with #2 clutch housings, don't think I've seen a #3.
These transmissions can be expensive for an OD, but deals are out there. The direct drive ones can be pretty easily found for much less money, often cheap enough to buy an OD gear parts kit and upgrade (but not available for the 9 speed).
Find out what SAE housing you have:
http://www.foleyengines.com/FreeTechnicalInfo/DrDieselShopKinks/SAEBellHousings.aspxThat'll get you thinking,
Grigg
