At one time there was a firm that would modify the overdrive tranny by adding a different rear bearing retainer housing so it would mate to the torque tube. At the same time, I believe the torque tube itself needed to be shortened. The book "How to Restore your Chevrolet Pickup" has a chapter on this now unavailable service.
Bolter Kimo scored a "Truckstell" overdrive unit which I believe is a form of auxiliary transmission in addition to the stock unit. He has a thread here on that.
Probably the best bang for the buck is to go open driveline with a T-5, or if you want to keep the torque tube get 355.1 gears from Patrick. We've beat this topic up quite a bit here if you do a search there are some good reads.
