intake/exhaust mounting, what am I missing?

are you missing your hot box gasket? fits between intake and exhaust at the heat transfer box
some of them also have a metal shim to direct the heat and facilitate the use of a flap that moved by a heat sensitive coil spring to basically switch the heat transfer box off and on, and you use 2 gaskets and the shim....but maybe that's just australia

either way there is usually gasket material between the inlet and exhaust at the hot box section


HP Premium Graphite Manifold Hot Box Gasket : suit Slant 6