I learned the hard way recently about stainless steel bolts in aluminum. What worked for me was PB Blaster and patience. A lot of patience. Of four, one took a half hour and one took three days. Just keep soaking it, then waiting and turning just hard enough to see if you can get some movement. If not, wait and try again. Once you get any movement at all you have it beat - but still, go slowly with more PB Blaster. (Be careful with heat, I understand it can harden the steel.)
PCS 31 #28
I find a manual impact wrench, the type you strike with a hammer can be a godsend when working with frozen bolts. An electric or air powered one would work too, but be sure not to go too hard or you may spin the head right off. You can pick up the manual one at a decent tool supply shop for 20 bucks or so, good tool to have. Oh, and lots of oil for days ahead of time.