Theirs were my first. Alumi-Koted too.
Went in OK (67 A-body), but trying to tighten the bolts to the head, on both sides the tubes near t e collector contacted the chassis.
A couple weeks later my friend's brother is putting a non coated set into his full size Blazer. They don't fit either. But he was more determined than I was and exhanged them for another set, which did fit.
All I can say is a if header assembly jig is so bad that they won't always fit in a full size Blazer, sumptin is wrong!