I just realized a possible explanation of what is happening. There were at least three counter shafts made for this series transmission.

I saw one that had a very low tooth count input gear. The first three gears (1st, 2nd and third) had corresponding tooth count ratio's to the gear ratios worked out the same. Fourth is always 1 to 1.

If you put the OD gear from a standard unit onto a gear set with a low ratio input gear without its matching OD gear its reduction would come pretty close to canceling the OD multiplication.

That explanation may be as clear as mud. But there is a combination of parts that can be put together that would get fifth to be pretty close to 1 to 1.

A salvage yard tried to sell me a box like that about 10 years ago. It was obvious that a shop had combined parts from several units to make one Frankenstein box.

The output shaft was a cut down passenger car shaft with short splines. I remember getting it, pulling the pan and noticing that it had a gear set that I had never seen before. I still don't know where the parts came from, but yes there is a way to have all the gears work but not get any RPM change from fourth when you shift in to OD.

Sorry it took me so long to figure this out. When I got the box it was sent from a yard in LA. What are the odds that it ended up going south and is the same one in your truck? It was an assembled complete unit and it had to go somewhere! I will try and find tooth counts on the early versions of our tranny.

Kevin