Good answers regarding parasitic loss.
But here's something else for consideration - neither the A500 nor A518 trannys are going to fit in your A-body trans tunnel without serious modification to the trans tunnel. No big expense if you can do that work yourself.
But if you have to farm out the trans tunnel mod work then you may want to consider a 200-4R OD trans that does fit in an unmodified A-body trans tunnel. Check out the various threads about this alternative.
The 200-4R has about the same parasitic loss as the 904, a better gear spread and a better 0.67 OD.
I'm putting a 200-4R behind my built 360 and going to run 4.30 rear gears. With the 0.67 OD it will be like having 2.88 gears when in OD for some freeway cruising.
