Header fitment rant, Doug's

The mounts the OP has look like truck mounts. I'd get a set of moroso solid mounts and never look back.

The part number on the box he posted earlier is correct for a set of A-body poly-locs...

But even if you look at the pictures on Schumacher's web site you can see the difference in the thickness of the mounts between the original style and the poly loc's
A67LA.jpg

A8EP.jpg

And the one's the OP has look even thicker than the one's shown Schumacher's site. Maybe that's just the picture, I don't know. Or maybe they tossed the wrong part in the box.

What I DO know is that everything else mounted to the engine depends on it being in a certain spot. That's especially true for headers. So the problem that has to be addressed here is the engine mounts. We know they're not the same as the originals, and we know it's causing interference issues.

The solid Moroso mounts look thinner
mor-62540.jpg
1971 DODGE DART Moroso Solid Steel Motor Mounts 62540

So do the plain old TransDapt original reproductions...
trd-4221_w.jpg
1971 DODGE DART Trans-Dapt Performance Motor Mounts 4221

I know I would try a different set of mounts before I beat up another header