So is the truck here in Wasthington, or is it still in California? I'm just picturing it blowing a T-5 apart coming up out of California into Oregon or on another of the steep climbs along I-5...
That truck empty represents more load than the T-5 was designed to handle. I can't imagine it lasting very long.
As for the Gearvendors approach, first its a lot of money, and second they are long, and I suspect you'd be moving the transfer case a long way back to make it work.
I'm not clear on how you intend to use this truck, so I may be way off base, but if it were mine I'd look at getting both the transmission and the transfer case from a modern (careful--my definition of modern includes things built before you were born) four wheel drive, and adapting both.
Since you have a crashbox, you don't have the later bellhousing even though you may have a later motor. The bellhousing that became standard in '48 will take pretty much any GM manual transmission that would make sense in that truck. The 5-speed manual with OD and a transfer case would be a good choice.
The reason I suggest using both transmission and transfer case is two fold. First, I've broken some old transfer cases, and been completely disabled as a result. Second, if you get them together that is one less piece that has to be adapted.
What are your expectations? This truck isn't going to fly down the freeway no matter what transmission you put in it... It's heavy, not areodynamic, and underpowered...
