It's not so much the size of the port that is the issue, it's the material around the port that the gasket seals on and where it is. The 340 manifold port drops far enough down that the EQ and MP R/T heads don't have any material for the manifold to seal to along the bottom.
Sorry about the search, I knew what I was looking for, but I could see how you wouldn't know what I was referring to.
10 Day Magnum Swap - '65 Dart 270
The above is a recent post, but has a link to a thread with a picture or two that should help some. Read beyond that post, too. Might give more info.
Didn't you open up some 318 manifolds? Seems like I remember you showing pictures where you ported the outlet or something.