Sulli ; From your description of the initial vacuum test , I don't suspect your engine chuffing is a result of that stuck manifold heat riser. Maybe , but ...
I feel it might be possibly more electrical ignition related. Double check for any goobers on the the little cam lobes inside the distributor, and look at any pattern on the faces of the ignition points (any arcing.) Free and smooth travel on the vacuum advance and centrifugal weight mechanisms. And get a new distributor cap before the huge amount of work (and expenses) of spitting manifolds.
That tiny 1hg dip on the vacuum gauge could be a minor sticky valve, dirty valve face, or more likely an electrical miss.
Your pics of that counter weight on the shaft looks to me like the manifold heat riser (flapper) is stuck in the "heat off" position ?
If it's stuck, I prefer the "heat off" and I would take time to get the engine to operating temperature (takes about 20-40 minutes at idle ~70degrees F). Your water temp gauge will show hotter water sooner than the actual temperature of the engine oil. Testing at engine normal operating temps for vacuum is better.
I have seen that butterfly flapper get really stuck from the inside of the manifold. Carbon and corrosion on the shaft. Once you do remove the manifolds you can likely unstick that valve once and for all; without the expense and labor of dual exhaust.
At this stage I would not twist or bang on the heat riser shaft too much that you would damage it. Keep diddling with it by drops of Kroil when hot & cold , and a gently side-to-side tapping and gentle rocking & twisting. May take a few weeks of engine heating and cooling cycles. Cheap to try.
Just my opinion, others may have another good direction to take.
