The issue might be the diameter of the manifold exit, which relates to the diameter of the flapper valve of the heat riser valve.
On our '66 Newport 383, the heat riser had "stuck" at the 1/2 place. No performance issues, which probably might have helped the exh crossover passage in the intake manifold stay open.
At one of the maintenance checks at the dealership, I asked them to free it up, which they did, dousing it with the Mopar Heat Riser Lube (penetrating oil with a slight amount of graphite in it). It worked freely for a while, but eventually stuck again.
Several years later, I asked about a replacement. They got the Chrysler service kit and installed it. I noticed that the shaft did not move freely, so the tech got a reamer, put it on an air drill, and reamed it to size. THEN the shaft went in easily and freely turned. SO if you get a Chrysler NOS kit, check and adjust this during installation. BUT as good as the installation was, the valve again "stuck" at the same 1/2 open place. In our temperate N TX climate, even in the winter, I could tell no difference in performance. Cold or hot.
It might be hidden by the counterweight, but the flapper valve's shaft runs in a brass/bronze bushing which is tapped into the exhaust manifold. That is a "wear interface", so it can be removed and a new one installed in its place. When replaced, by observation, the inner diameter of the bushing needs to be fine-tuned to the outer diameter of the shaft which runs in it. Just good installation practices.
I'm not sure how "worn out" describes a stuck heat riser valve, UNLESS they were seeking to sell something. FWIW
Just some experiences and observations,
CBODY67