lots of options given here... aside from a crack, i second the heli-coil 6mm repair. make sure you back the tap out 1/2 turn for evert 3/4 turn you make with it. it's tedious, but if you don't it comes out all choppy. i usually do about 2-3 turns, slowly, then remove it completely, clean the shavings off of it, then start over again. the first couple turns you make are the most important. if you don't back it off regularly, you'll wind up with a sloppy hole. Also, if the inserts are really short, i have no issue putting one insert deep inside the hole and then butting another up against it towards the outside. as long as you don't overlap them, you won't have any trouble. this means there's a full length of SS threads inside of your freshly tapped slight oversize aluminum hole. should be stronger than stock, believe it or not....
hope you're using an inch pound torque wrench on those head bolts, btw...