how much hp will a 8.8 hold?

350hp? An 8.25 will take that with no problem. Just do a proper rebuild and you should be fine. I wouldn't bother with an 8.75 until you get up into the 450-500hp range. Check the article:

Mopar Muscle Article

I am going a few steps further than the article went:

1) Welding the axle tubes.

2) Using Moser C-clip Axles.

3) Installing a post-97 SureGrip (29 spline vs old type 27).

4) Studding the carrier caps

5) Welding 1/2" angle steel to the axle tubes from the carrier to the spring perch, "just in case" I ever move to a mild stroker.