There are two belts for a 98 GP with the the non-*/c'd 3.8, a 93.5 inch and a 93.0 inch. The 93.0 is referred to as first design, and the 93.5 as second design. I do not know where the breakpoint from 1st to 2nd was, but I am willing to bet you need the 93.0" belt.
If that does not work, auto stores carry a sticky spray specificially to correct belt slippage. I would get a 93.0" belt if I were you.