A904-Need feedback on RUMBLING IN "D" gear

1971 Dodge Dart Slant 6 with A904 tranny. Just purchased vehicle and FINALLY got it running. Go to test drive so vehicle shifts in reverse strong and smooth. Shift to "D" gear to go forward and noticed that vehicle BARELY goes. Also there is a severe RUMBLING throughout the entire vehicle (Not safe to drive). Vehicle had been sitting for a couple of years and found out problem was there and previous owner replaced TRANS filter & gasket to TRY to fix problem. Please tell me what you think. Am purchasing a used unit regardless, removed from a good running/driving vehicle that has been sitting for a couple of years. Could not pass up do to PRICE from a great "A bodies" member. THANKS