the stock motor thing is totally unofficial.rules are somewhat flexible, but generally speaking, from the throttle body to the oil pan you don't touch the motor.
no rockers, no cam, no intake, no heads, (some people would even say no valve job or race valve job which I think is a standard 'cheat')
otherwise, go gears,pullies, exhaust, maybe an electric fan + slicks and have at it.
probably 4.30 gears and a 28" slick would work best. Expect to cut a 1.6/1.7 60' to get there!