Well first, put away the impact gun before you break something!

Next, is there a lot of rust and corrosion present? If so, saturate the area as best you can with your favorite rust penetrant (WD-40, Liquid Wrench, etc.) and do it repeatedly over the next several days or a week. (patience is a virtue, remember) You need to give it every chance for the penetrating oil to wick it's way up the bolt shaft and into the threads. Instead of the impact gun, buy a 1/2" or 3/4" breaker bar, a 6 point impact socket and strong piece of steel tube to slip over the breaker bar to give you more leverage. I used an old motorcycle front fork tube. Living in So. Calif., mine didn't have any rust present but it still took a good strong pull to break them loose, and they were very tight for probably the first 2 or 3 turns.

The bolts are installed with something like blue loctite, a thread lock compound. This stuff goes on as a medium viscosity liquid that fills in the gap between the internal threads in the frame and the external threads on the bolt. In the absence of air when the fastener is tightened the compound hardens and locks the bolt in tight. When it's feasible you can apply high heat, as from a propane torch, and the loctite will weaken and break down. Unfortunately, using a torch in this situation is not the best idea. I suppose you could try directing the flame at the bolt head and see if enough heat soaks into the bolt to affect the threadlock, but I would use extreme caution as the gas tank and fuel lines are in the vicinity. Keep a fire extinguisher handy and your insurance premium paid up!

After soaking in penetrant for a good long while, use the breaker bar, with extension if necessary, to attempt to loosen the bolt. Avoid sudden jerking motions, as these could twist the head off the bolt or even move the car off the jackstands. If you can get it to move even a little bit, rotate it back an forth a bit to break loose the threadlock and then attempt again to unscrew the bolt.

Once you get the bolt out be sure to renew the threads with a bottom tap or thread chaser. On reassembly be sure to use new bolts.

Gary