Exhaust leak at the choke stove

By your description, you're working with a '70-'72 exhaust manifold, the kind with the open-hole choke stove. '60-'69 and '73-up manifolds have a cast-in pocket with no path to the interior of the exhaust manifold.

The '70-'72 type is prone to exhaust leaks. The stainless steel choke cup warps, and the correct gasket is no longer available (the V8 gasket is, but it's not the same/doesn't fit), so your options are either to seal up what you have or swap to a different manifold. If you decide to try to fix what you have, you'll have better luck making a flat steel blockoff plate about 1/8" thick of the correct shape and size; that way you'll at least have a sturdier clamping surface for your homemade gasket. Also this way you can have a solid piece of gasket material instead of a much flimsier open-centred ring of material. Use Remflex material to make your new gasket. Discard the original choke cup and thermostat and plunk a № 1231 Electric choke kit on top of your new blockoff plate.