I had a a Challenger bblk, I had to take a belt sander to the head flange to get it to fold under, kinda like this one.
That header was fkd, but I got it to fit.
The old days were a nightmare, no 2 headers were the same, hand welded, flange was like a mountain range, weld/ file ever set.
At least now the use a surface grinder for the flange.