I bought a new Dorman exhaust manifold and when I compare the heat riser valve to the original heat riser valve there are some difference in size. The original valve does "cover" and deflect the exhausts from the intake heat box when the valve is heat off much better than the Dorman does. Dorman's heat valve looks like it would escape exhausts past the valve to the intake heat box easier. Is this a concern?

I'm going to run a Dutra Dual manifold so I'm going to cut and modify the Dorman manifold as needed. I will remove the heat valve when I'm going to weld the Dorman manifold and I'm thinking I can easily modify the heat valve so it cover/ deflect the exhausts like the original. I bought the Dorman manifold to get a working heat riser valve so I just want it to work as intended.

Don't worry about it -- the Dorman manifold has the later-type valve, and it works fine even though it looks a little different in the details compared to the early type. You do not need to modify anything.
Not wise to remove the valve -- that will tend to ruin it. Leave it alone, leave it in place, let it do its job.

It is very much worth your while to get the good gaskets for the manifolds-to-head and intake-to-exhaust junctions.

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Too many people who were born on third base actually believe they've hit a triple.

Thanks for your knowledge Dan.

I'm only going to remove the valve so I dont damage it when I preheat/weld the manifold after I cut of the front bit of the manifold. Then put the valve back after the welding. Or do you mean that it is not wise at all to remove the valve because it may damage something?