[Dave Colangelo]

Heating the head of the bolt wont do much but soften it, its the flange you want to expand a bit to allow the threads to come out easier, try heating from the backside. Id stay away from impact tools on this and stick to PB Blaster/Kroil paired with a breaker bar instead with smooth force applied, the allen bolts are just a bit to prone to stripping for an impact tool to be the right thing here. The thread holes come out the back of the flange so you can also spray the PB-Blaster from the backside and get it in there. Tapping with a flat hammer can help break light rust and seizing when needed.

The flange (on both sides) is steel.

Regards
Dave