2nd the penetrating oil, also could try some application of force in the tightening direction first, just a bit, to try and break the threads free, then back the loosening direction. Keep a lot of downward force on the breaker bar head to keep the bit as deep as possible in the bolt, like maybe get a helper to push down while you do the turning, and buy one better grade T-50 torx bit, somethin at least Husky er Crapsman(although I break them, too). If the bolt gets rounded out, you'll have weld a nut to the head of the bolt to be able to get it out, and welding that nut on will require a lot of prep to protect the rest of the interior for ten seconds of zap.