I will assume that you pulled your stock headers off to replace them with aftermarket.
To make a drill guide, You will need to cut the mounting flange, (to the head) off the header.
Then order a metric drill guide that fits into the hole of the flange where the broken bolt is.
McMaster-Carr
Grind the bolt flat,.... bolt on the flange with the drill bushing installed and use the correct drillbit that fits the bushing hole.
Drill out bolt. Then easyout the rest. I use left handed drill bits as 99% of the time it screws them out on their own.