A 904 is simply the fastest stock based trans out there. If you change over to a 2.28 low gear and the car slows you need more rear gear ratio to make up the difference. Same thing with powerglide. If your engine does not make enough torque to pull the higher gear you will not like the glide.

Powerglides are fine but if you are now running a 3.91 gear you will probable need a 4.30 to work with the glide.