Scott, just checked our 66 with thermactor and the "smashed" part of the heat tube does in fact give a little extra room for the air manifold that goes in the exhaust manifold. I am not sure if one of the round tubes would work with the log type air manifolds but it would certainly be close without the flat spot. Just for information our car has a scheduled build date of October 30 (1965), built in SJ, 289 two barrel with C4, PS and AC. Hope this helps in some way.