Just go to any parts store and ask for a radiator for a '98 Monte w/ 3.8. It's 1 3/8" thick and it's what I'm running. With my discount at AutoZone, I picked mine up for $200 otd and a lifetime warranty. Plus, they stock it in the store.

FWIW, my car never goes above 190* on the temp gauge, it's amazing at keeping the engine running cool.