Exactly what Bill said.
The only difference I've ever noticed with old manifolds is that over time the block and manifold can warp a little with heat causing an exhaust leak. Lay a straight edge along the manifold connection on the block and if there's a gap, you might have a leak with a new manifold, or it could be planed down to flush again.
I had this happen on an old 390 and went with very thick copper exhaust gaskets to fix the problem since it wasn't worth the time an involvement in fixing it in other ways.
Jeff
