I just gave up and used copper RTV on the uneven collector pipes on a flat 4 engine. There was no way to properly align the left and right headers to the collector besides just fucking cranking on the bolts as a clamp held them in alignment.

I felt shame for that one, and it’s off to the inspection station Monday now that it doesn’t leak. But you can see the Rtv sticking out. I give it a summer before it fails again. At best.