Missing 3rd on 904 trans

AHA, so it's First gear that you are missing.
Well then I can tell you right away, that since you have reverse, the band is working fine. And, if you can take off in "drive", then the One-Way clutch is working.
So that narrows it down to an oil-pressure issue
Have you tried a 2-1 forced downshift?
If the trans will Not downshift into manual Low from Second, then I would suspect a stuck 1-2 shift valve.
But if it does, then I'm going with the governor circuit.

Now, I gotta admit, that I have played with these torqueflites not so much,and am not all that proficient in diagnosing problems. I can build them no problem, and I can kit them, and I can mod governors. All of that cool stuff. And I know my way around the flow-charts.
But hi-strangeness is still a bit of a guess for me.
There are a couple of guys here on FABO way more proficient than me. One of them I think, is @CudaFactHackJob , so this will page him.