Give ARP a call. I had a couple of unique bolting situations and gave them a call - they came up with bolting that worked and surprisingly, at a very reasonable cost.
If calling fails, I would first try to grind a relief in the header flange using a 90* die grinder and a carbide burr. I would as a fall back position, maybe install a NEW GM bolt.
This is one the die grinders that I have - sorry about the thumb nail but I'm doing a house cleaning on Photobucket: