My GF's Duster 360 is slow as hell !!!

Kicking an engine like this down at cruising speeds (55/65 mph?) should get you the feel you are looking for even with a 3.21. From a dead stop a lower gear or shorter tire would be a benefit. Maybe you have a timing or advance problem. I once put a Cleveland into a Maverick Grabber with a 2.88 and it would smoke an L-60 untill it was destroyed.