I think it was the later exhausts where the choke pocket connects to the interior of the exhaust, because a guy posted a few months ago about his 1974 slant leaking exhaust from there. He lives near me and I later saw his engine when he bought a carb from me, though the leak was fixed. I think if the OP finds a 74+ choke thermostat, it will plug the hole. I don't know what gasket or sealant the factory used there, but hi-temp silicone might work. My 69 slant had a closed choke pocket, as does my 64 slant.