heli coil

I have a problem with my 270. One of the injectors popped out and stripped the threads. Tired m6 helicoil but the original bolt holes to big for m6. Can i go m7 or even m8? Is there enough surround metal to be able to drill n tap an m7 or m8 helicoil. All help appreciated

Well i think i`ve found my rough running problem, the very 1st injector popped out yesterday , inspected today it has took the helicoil with it, looks like a bad repair by someone, the injector itself has a long crack on the bottom section , i`m guessing this has been the cause of it running...