You're on the right track. Using a mock-up head is ESSENTIAL IMO. The flanges on the repro headers are thicker to a different degree on each flange. That's why just milling the flat side won't do. They have to be done individually. I did mine with a mock up head on my bench. I was extremely careful not to overgrind. I'll bet I put them on, measured, removed and grinded about 20 times for each header. I could only imagine the frustration trying that with the head in the truck.
