Once I got the 348 out of the truck, and the 409 onto the mounts, I was happy to see that that the exhaust pipes appear to line up to the manifolds.

BUT the flanges are different. The 409 manifolds are 2.5 inch and the 348 manifolds are 2 inch. The studs on the 2.5" manifold are farther apart than on the 2" manifolds by a tiny amount and I could possibly file the holes on the 2" flange a bit to get the studs to fit, but that would be a chore. And I would have to figure out something for a gasket to span the difference.

Question 1- Could I put the 348 manifolds onto the 409 heads temporarily, to drive the truck to the muffler shop for new flanges? (I'm thinking that I could take the new manifolds along and have them installed there.)

Question 2 - Once I do get to the muffler shop, how important is it that I replace the nearly new 2" exhaust pipes with 2 1/2"?

The 348 is a '58-61 250hp.

The 409 is a '63 QA