As you know everyone has their opinion. The nice thing about Mopar cars is they can be made to run and handle sweet using stock parts. The 904 is no problem. You can change the fluid and filter, replace the 2nd gear band lever to the 4.2 that the 273 Commando used
BAND LEVER - A&A Transmissions , adjust the bands, and up the pressure while you have the pan off. If you do not have partial kickdown you can add that also through A & A Transmissions,
727 3-2 Kick Down Module (1966-69) - A&A Transmissions . Make sure you can adjust the kick down when you swap to the 4 barrel carb. Try this first before you get a convertor. What brakes do you have?