Manifold Heat Control Valve - stuck close

Tom, I'm going to have to keep disagreeing with you on this one. Something you guess you think you imagine might could've happened doesn't count as real life experience. In real life, this fire scenario you're afraid of just doesn't happen. Think how many gazillion Slant-6 vehicles have been in service all over the world since 1960. If your scenario happened to even 0.1% of them, it would be a sticky at the top of every Slant-6ers mind.

In the real world, here's what actually happens: when the fuel boils out of the carburetor on a Slant-6 engine, the vast majority of it exits the carburetor through the nozzles and vents located in the throttle body and venturi. This floods the engine, making it very hard to start and diluting the oil. Those are bad things, but they're not fires. A small amount of the boiling gasoline can escape the external bowl vent and make a drip or two below the carb on the intake manifold (and if the carb's got wear at the throttle shaft, a small amount of that internally-boiling fuel can seep out through there—nowhere near enough to flow down to the exhaust manifold, and even if it did, it wouldn't catch on fire from the heat, it would simply turn into fuel vapour and stink up the vicinity of the car.

There are plenty of good reasons to put and keep the heat riser valve in proper operating condition (and use the correct thick carb base gasket, and do the Fuel line mod). Avoidance of an engine fire is not one of the reasons.