Can you get an open end wrench on that bolt & move it a fraction of a turn at a time before you reposition the wrench? Loosening all the other bolts then seeing if you can tap the header rearward to gain a hair more of clearance between bolt & tube might help you unscrew that bolt. Also unscrew the bolt then tighten it back up (just snug) then loosen it even further. EDIT you will need to move the flange out as you unscrew the bolt. Have all the other bolts out so the flange can move as you work on the 2 problem bolts, unbolted at the collector also so the header is (almost) free