Looking for some advise on the heat riser in the big block HP manifold. Thinking about either tac welding it open or just removing it completely and welding up the two holes. Advantages or disadvantages? Any input is appreciated.

With my HP manifold, I just had the heat riser removed and the holes welded up. Didn't want any restrictions or any worries from it. I'm sure it would be fine also to weld it open if you have seconds thoughts about removing it. As long as there are no restrictions in the exhaust!

Anything special needed to weld the holes in the manifold?

This is my plan;
The_look

I just did a manifold for my turbo slant six. My machinist just threded the holes and used plugs. No risk of cracks that way. Maybe you can thread that stud in there.

On my HP manifolds I wanted to keep the stock look. I removed the disc (valve) and ground the shaft flat so it presented a minimum of surface area to the exhaust flow. If stock looks don't matter, Killermopar's threading idea sounds like an excellent option.