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.

I'd remove it and weld or braze the holes shut. I have a set of BB HP manifolds done like that and just did an HP 340 manifold. On the 340 manifold, it looks factory from the outside, but there's nothing inside the manifold anymore. Just cut it out, welded it shut, and made it look like stock. Had them high temp powdercoated too.