I use the 3/8" flanges that Speedway and many other companies sell with their header kits.... Never had to do more then elongate a hole or two. Building headers is labor intensive enough without wasting a bunch of time building flanges!!!! I'm with Jerry, maybe if I needed 500 sets, but for only a 1 time deal???? On any unique engines or unusual tube requirement, I just get the appropriate header gasket and take it out to my pal Brian at Laser Cut.... Times gotta be worth something, or at least better spent on the rest of the car???