A little off topic, but my experience with current replacement heat riser valves for both my '66 Chevelle and my '69 RS were dismal. (Tried parts both from a major auto parts dealer as welll as a major restoration parts supplier).
In all cases, and after a relatively short period of time, they would bind and stick - often in the closed position. There is a "donught" that you can purchase that has the shape of the heat riser minus the internals and external arm. It installs in the place of the actual heat riser valve so the exhaust will connect correctly to the manifold.
For the relatively few hobby miles that I put on both cars each year, I haven't had any driveability or warm up issues by not having a heat riser valve installed. And by not having them installed, I have eliminated a real nuisance - sticking heat riser valves - often in the closed position.
Also, for me, having the heat riser installed from an originality perspective was not important.
I just don't believe that the pacific rim pieces available today are of the quality necessry to ensure long term service - but just my humble opinion.
