I read somewhere that someone had this problem, they had a machine shop make them another bolt with a 7mm thread instead of the 6 and wound that in there, now I'm not sure if they tapped it first, but you can actually turn the bolt thread itself into a tap for ally by putting two cuts in the thread wit a hacksaw, you generally can't stop a leaking gasket on these by tightening up the bolts once it's started leaking, I would imagine you would still have a leak even if you did the 7mm mod
oops, just seen the date of post, well hope this helps someone else, would like to know how op got on