My Buddy is having the same problem with an old truck 727. He never ran it but had the transmission rebuilt and when he went back to the transmission guy and said it was out of an old military truck the transmission guy told him they had lower planetary gears. I never heard of such a thing but can it be true?
The Tflites in those old army vehicles may have lower gear planetaries but the rear gear in those rigs are in the 5's or 6's (5:13 etc.) Wide open they only go 45 or 50. It's the rear gears. Like old farm trucks. Same MO.