Yes, that's the problem at the bottom of the port with stock replacement magnum heads.
The 340 manifolds are a large LA port and the magnums are a small port head.
I made some 16 gauge block off plates like Dion R made and they leak slightly around both sides of the plate. it's not much around each port but you multiply it times 8, it's annoying.
I talked to a friend of mine tonight and he has a machined straight edge and I'll have him
check my manifold for flatness tomorrow.
The exhaust port on the heads from Hughes are like mine, not enough pad under the port to seal the 340 manifolds. I think the 273/318 manifolds would fit fine, but who wants those on your engine.