904s are not a weak point in most drivetrains if built with even minor upgrades. Also most cars that are swapped from a 727 to a low geared 904 are very noticeably quicker, I would not hesitate one bit for most cars to swap in a 904.

Lock up was totally hydraulically controlled till about 89 then they added a solenoid to de-activate it but not activate it, they still used hydraulics to turn it on but the electronics could overide it usually for cold or heavy throttle driving.