Isn't this more of a problem with the 727's? I know my 71 340 was always a little hesitant in reverse when it was cold or I hadn't used the car for a week.
It may be more pronounced in 727's but can happen with 904's too. It's usually because the stator and/or rings or the front drum are worn and lets the pump drain back. The 904 is the same design as the 727, just smaller
In the case with the OP I agree with AJ/FormulaS that the issue may be related to the stator and/or front drum
