I just learned something that might be useful. I am reinstalling a 1968 350 in my 1968 Skylark. I am using the 2-1/2 inch TA Performance down pipe. Due to other modifications, I am not using the stove nor the heat riser valve so I gutted the valve, plugged the shaft hole and bolted it to the flat flange on the bottom of the manifold. I did this because I needed the nipple on the bottom of the heat riser valve to accept the female flare on the top of the TA down pipe. I have another 350 laying around that I should have looked at sooner. Its blue with HEI and lots of emissions stuff but I don't know what it is out of. Anyway its manifolds are made opposite the 1968 versions. The left hand has the flat bottom flange and heat riser casting number B-1246669-1. The right hand is slightly longer than the 1968 version and includes the male nipple as part of the manifold casting number B-1246656-1. It would have made a cleaner installation had I used this one on the right side of my 68.