I would soak those rusty nuts of yours in a good quality penetrant. I prefer Kroil while many people swear by PB Blaster. An air impact is not the tool for the exhause studs. After soaking for a couple of days use a socket to try to loosen the nuts. Some people say they like to try to tighten the nut slightly if it doesn't want to loosen on the first try. Whatever works because breaking them off is a bummer to be avoided.

Actually I've never had much of a problem removing the exhaust nuts. It is replacing exhaust studs that gives you gray hair :-)

Randy