[Steeveedee]

I disagree with Lars from your link. There is very little pressure from exhaust gases in the first place, and once the heat riser opens, there is even less. This design is a compromise reached by engineers working on making the vehicle work in any environment, within reason. Depending on what your guy did to close off the ports, you may well be done with trying to stop the heat flow (it goes up hill, so that area is still getting hot). If you want to keep the manifold, ask your guy what he did. It may not require any more work.