I have a 63 gt with slant 6 and want to drop in a 318/904 combo. I don't know exactly what tranny I currently have but I believe it is an A727. besides the motor mount would I need any other modifications (rear end, drive shaft etc...)
Will need a 111-inch wheelbase A-body drive shaft for a 904.
Not sure when it started, but by the '66 models, the front flex joint, ball & trunion, was replaced by a Spicer type universal joint. Your 318/904 combo probably has the U-joint, so the drive shaft will need to come from a 66 or later Dart (except Demon/Dart Sport & 66 Dart wagons), Swinger/Scamp, 73 or later Valiant 4-door.