prepping drivetrain for the new engine- 8.75 & 904, need advice

I would leave the rearend housing alone. You should be able to get wheels with enough backspacing to get the maximum tire size on there. I still doubt you'll be able to get a 325 tire under a scamp with leaf springs. you'd likely have to do a full tub and backhalf for a 325.

Contact www.coperacingtrans.com about a 904 holding up to that much HP.