You are correct about the A500 fitting in A bodies WAY easier than the 518.
I am using a hydraulic/electric shift control with pressure switches and had to very slightly ding the hump so the fitting coming off the governor port didn't touch. (a half inch deep x 1 inch round ding)
The cases of the 904 and A500 are the same size, but different internally and the same with the 727 and A518, and the OD unit on the A518 is bigger as well IIRC.
Yes there is a trans crossmember adapter for 904 to A500 or A518, and replacing the speedo cable with an 80" is the best way to handle that part.
The cooler lines and fittings are different as well, but the speedo gear drive and gears from a 904 fits the A500 and A518 both, but don't read correctly when put in the A500 or A518.
I needed a smaller (faster) gear to get it close.
The A series have 3/8 cooler lines as opposed to 5/16 on the non OD's, and they connect to the trans differently (pop in, instead of threaded fittings)
I used compression union/NPT fittings in the case for new 3/8 lines.