I've torched lots of bolts like you have, and a quick pass with a threading tap usually cleans the threads up. If they are chewed up from drilling, a Helicoil might be necessary.
A tip--set the head on a flat surface outside, gasket surface down so it kinda seals on the table, and run a garden hose in the coolant passages to cool it while you gas-axe the bolt. Messy but it works. Duct tape seals some other leaks and keeps the water where you want it.