I have `99 GTP rims on my `94 Cutlass. I had to use 1/4" spacers to clear the front calipers but IIRC the rears were fine. If our cars are measuring 115mm I recall reading years ago there was a desirable Mustang rim in 113.5mm that was close enough to work. I never mounted them to see for myself though.
?
the bolt circle is 115mm, Gen 1 & 2 are the same circle, one CAN get Gen 2 wheels to bolt to the Gen 1 suspension but one needs (ugh) a minimum 1" spacers to allow the needed clearance for the calipers.
I'd be looking into why the OEM brakes are so weak. I bet you don't need "upgrades", you need "repair".
Partially-seized calipers, sticking caliper guide pins, crappy, bottom-feeder pads, failure to flush ancient brake fluid using a scan tool to activate the ABS, perhaps a failing power booster--or not enough engine vacuum to work it, etc.