I had the same problem on my '79, Steve and I used Heli Coil and Loctite. The studs are 6mm on both sides. My concern with 8mm studs would be, that there is not all that much material in the head around the studs to enlarge it to 8mm. You would also have to drill out the header flanges. The holes are only about 7mm.
You could probably get studs that are 8mm in the head and 6mm for the flanges.
I would also be concerned drilling and tapping new threads with the head on the engine. Very poor angle to do a good job