4-Speed 904 For \6?

Yes, an early 904 with an A500 overdrive unit.
Basically you would just be using the /6 case and the A500 insides.
You would have to use the output shaft and planetary set from the A500 as well as the valvebody with the OD control valve assy.
It's all fluid and valve controlled, so there would be no electrics involved.
There are obviously plenty og other options, but if you want to keep it all mopar then it could be done this way unless I am not remembering something important.

I also don't remember if your year of trans has the rear pump in it.
If it does, we can forget the whole idea.

Even if my trans has a rear pump, I could find a later model \6 904 as a doner for the case.

Suposedy they used a rear pump through '65 but I can look to see if it has a splned output shaft. If it does hve a splined output shaft, it does not have a rear pump.

Is the A500 as strong as the 904?

Does the OD unit make it weaker?

The GM 200R4 option is interesting too, but I would rather stay Mopar & not use adaptors.

Also, does the A500 have a lock-up convertor?