PB Blaster is my definitely my penetrant of choice... but sometimes, only brute force will do.
If you have access to a propane torch (like the little Benz-O-Matics), you can apply heat to the bolt, and try and loosen it. Try and get it cherry hot, if you can. You can pick these up in a kit from your local hardware, etc., and replacement cylinders can be found anywhere - even at WalMart (they probably sell the kits, too).
I haven't seen a bolt on a GMT360 yet that I can't get off with a 1/2" breaker bar, if I can get access to the bolt. Sometimes I have to put a length of pipe over the bar, but the bolt comes off.
If your calipers have never been off, you'll find yellow thread sealant on the bolts. That's GM sealant, and it is pretty strong stuff. Not as bad as red, but it's close.
If none of that works, you mentioned an impact. If you have one, but it doesn't fit in the space, you can get wobble extensions in various lengths, swivel sockets, etc., that can help.
Depending on how heavy-duty your existing gun is, you can try and reverse direction -- after trying to loosen the bolt, reverse direction and give it a couple of shots. Back and forth a few times, and it should move, eventually.
Your location is listed as CLT, which I'll guess is Charlotte, NC, as that's the airport code. There are a ton of Harbor Freight locations in that metro area. You can get all of the extensions, sockets, etc., there, if you've never been to one. And their hand tools are guaranteed forever, no questions asked. I've never broken a socket of theirs (well, I did, once... using chromium sockets with an impact gun is asking for trouble)
Since these are the caliper bolts, that reminds me of what I use, especially for one of the two, as it's a bit obstructed.
I have a set of these (and I didn't have to pay $32 for them, either...sorry). The 18mm you need is in the set. I think you'll also need an extension - maybe a 3"-"6". But these work for me on calipers. Even the ones on my full-size pickup (the torque on those is spec'd at about 225 ft/lb, while yours is spec'd at about 168, IIRC - ?) You can use these with a breaker bar, too (by hand).
Amazing deals on this 7Pc 1/2In Dr Metric Impact Swivel Socket Set at Harbor Freight. Quality tools & low prices.
www.harborfreight.com
If you're *still* having trouble, HF also sells an electric impact gun that has nearly 1200 ft/lb of loosening torque. I have one, and it's a good tool. But it's a bit heavy, and bulky, too. Extensions are your friend, with this. Do not put a 4-way universal (even an impact-rated one) on the gun -- the torque will rip it right off as soon as you pull the trigger. The swivel impacts are fine.
Finally, if *none* of that works / suits you... take it to a shop and have them loosen the bolts with their pneumatic guns. They'll charge you a few bucks for the privilege.