904 Conundrum

So the 904 in the '73 Valiant was toast, or so I thought. I was missing Reverse and Third gear, the most important two. I knew the 904 in my '72 Valiant was good (both are 225 SL6ers) and the '73 was my daily driver, so a quick swap fix, right?
:angryfir:
Wrong.
:banghead:
The swap was more of a PITA than I thought it would be, but I finally got the trannys swapped, supported, sealed, tightened, and filled with fluid.
:hello1:
And I have the same problem...kinda.
:scratch:
Reverse takes a little bit longer to engage than I'd like and there is slippage between the gears. Sometimes a neutral slip, sometimes a slip to the next or last gear.
:wack:
I know little in the ways of automatic transmissions and what makes them know when to do their thing, or what interrupts that sequence.
Can someone please provide a light into what the heck is, or could be, going on?
:prayer:
The car afflicted:

she misses being driven...