904 Trans Struggle Bus

New to the Mopar world, my teenage son drug me in kicking and screaming with his inherited 69 D100 and recent purchase of a 66 Dart. The issue is related to the Dart. Slant Six car in pretty darn good condition but when we parked it for a few days I noticed a decent puddle of trans fluid so we got a gasket and had him pull the pan to replace. When he went to bolt the pan back up turns out someone got overly excited at some point and mangled the threads on 10 of the bolt holes and had put in heli-coils to try and save it and well…. Not good. Me not knowing anything about these cars and the particulars bought another 904 that turned out to be a small block bellhousing, was super cheap and we had considered swapping in a 318 we have so I figured no harm no foul. Found another slant six bellhousing 904 that had come out of a running driving car very recently and after carefully ensuring each bolt would hold 15 lbs of torque we set to getting it ready to go in the car (some of you probably have already guessed where this is going). Lesson #2 learned, the input shafts on the trans are different for the later 904’s and of course what we have is a later model so the torque converter would not go on. I had bought time-serts and contemplated fixing the original trans but would much prefer a non mangled case. Now the question is, can we strip out the newer case and get a rebuild kit and put the parts from the original trans, most importantly the pump housing and input shaft, into the newer case? I also read that the neutral safety/backup light switch is valve body specific so would have likely had issues with that as well. If moving things over is a feasible path is there a preferred vendor for a rebuild kit?