727 -> 904
I understand where your coming from. But I think if you figure up things the cost will be close no matter which trans you choose. You say the 904 will cost you $300 plus need rebuilding adding $150 plus needing driveshaft work adding another $150. That brings the total to around $600. Add $50 for flushing the torque converter cause any torque converter should be flushed out if the trans. has been burned up that it was in. Now your at $650. Stay with your 727 and rebuild it ($150 for a kit cause they cost about the same as a 904) and put in a new converter and deep pan (you said $500 for them) and your at $650. Same price and in the end you still have a new trans with a stall speed converter and deep pan and it's the heavier duty 727.
Don't get me wrong I'm not trying to make up your mind for you. Just giving you something else to think about.