My tip, if you are thinking about heli-coiling, is to not use heli-coils, but use time-sert. They are a much higher quality product than heli-coil; a solid bushing insert that is thinner, so you won't need to drill out as large of a hole. The set will come with the proper size drill, counter-boring bit that will cut the proper depth counter bore at the outer edge of the hole, tap, and the installation tool that rolls bottom threads. You will need to figure out what depth bushing you want and order that.
++ TIME-SERT Official Threaded inserts for stripped threads, blown out sparkplugs,
Also, use anti-seize on new bolts to prevent this from happening again.