Question about drilling and tapping a head??

[dznucks]

I have a helicoil that has broken off in the passenger side head. The bolt is the lower alternator bolt and the helicoil pulled out the threads in the head. There is still a portion of the helicoil in the head and i am able to grab a few threads but it is working itself loose.

I want to replace the helicoil with a threaded insert since the original oversized threads in the head are gone. Its a 3/8-16 helicoil and the head i am guesing was taped at 7/16. The hole mic'd at .397

My question is...do i have enough metal in the head casting to drill the hole out to a 31/64 (.484) without weakening the head? the external threads of the insert are 9/16-12.

its a '69 with a jasper 350. jasper helicoiled every hole in the block.