Tried today to get a left-side HP exhaust manifold to fit on a 906 head that's going on a 383 rebuild. Casting # 2951865-2, dated 0-7-70,is supposed to fit 70-71 B bodies and C bodies. The manifold would not go over the studs. I had to ream the bolt holes slightly on cylinders 1 and 3 to get it to go on. Manifold was not warped, checked straight. Bolted up OK after the mod. Anyone else ever had something like this happen? The manifold in question had been off an engine for at least 25 years or so.

I had it happen on one set of H/P manifolds I had for my 383. As stated its usually at the front. I ended up selling them and using headers so it wasn't a big deal. Mopar guys are lucky as its not a super common problem. Try installing the old Chevy ram horn manifolds on a small block sometime. I used to have to use a ratchet strap to drag the front cylinder forward far enough to get the bolts started!

Did you try another manifold ? Does the head have the original holes or are they heli-coiled ?

You fixed the problem , I wouldn't worry about it past doing a gasket match , some manifolds are really bad at the port opening on the end cylinders .

I tried the only other left-side HP manifold I had, one off of a 68 GTX...it fit just fine. I also tried the 71 manifold on a couple of different heads, 346 and 452...it wouldn't fit them, either.

You said it checked straight, I'd confirm that as SOMETHING had to be significantly warped & most people dont use a gasket since Ma didn't but there's not a problem with using a gasket or even just some sealer. You just dont want an exhaust leak but you'll know if it is something to be concerned with