So, I've got an a904 in my coronet. The car will drive in reverse, but will not drive forward (it just revs like it's in neutral). I've got another a904 that can be installed, but one of the tabs for the converter on the front pump is broken off. So, my question is, what do you think the chances are that the front pump in the transmission in the car is bad, or could I get away with throwing it in the other a904 to have a driving vehicle? Trying to avoid putting too much money in the transmission so I can save up for a manual swap.

I'm assuming both 904's are '68 & up so I believe the front pumps interchange. The issue that comes to mind is when the lug broke in the spare trans was any damage from debris done to that trans? Otherwise I'm confident you could swap the good pump in the trans that is in the car to your spare trans. EDIT I ain't sure what band/clutch/1 way overrunning clutch etc is on or off in drive but you might pull the pan as it might be a strut but someone will chime in with what has to be operational in forward drive

If Reverse works the pump is OK.

If it won't move forward in Drive try putting the shifter in "1"...if it moves, the sprag is gone but if it won't move forward in either Drive or "1" the rear clutch is the problem.