manifold heat control valve

Even if the shaft turns, the internal plate could be corroded away so it doesn't function. The one on my 69 Slant Six was frozen stuck, so when I had the manifold off (for other things), I removed the heat riser. I wanted the option of installing one someday (if new parts ever appeared), so I used a bolt and nut to seal each hole. I used the the type nut with a round washer head and grooves under the washer, with the nuts inside. They never came loose in 10 years of driving. You might use stainless steel, but a little corrosion might be good to keep it from unloosening. You could use a single bolt thru both holes (my initial solution), but don't overtighten or you might crack the manifold. That is one way to do it with the manifold on the car.

I drove that 69 Dart for 2 winters in South Bend, IN and I don't recall unusual problems with it heating up, even on -10 F mornings. If you someday switch from a carb to TBI, I don't think those need a heat crossover. For sure MPI doesn't. I understand the main reason for the heat cross-over is to prevent ice buildup under the carb, meaning steady-state driving in cold and damp climates. The "heat riser" valve aids in the secondary benefit of a faster warmup, but isn't essential. What affects things more is if you block off the cross-over, which many do in V-8 engines for racing, or which slant-sixers do by installing headers.