HOLY FRIGGIN' MOLY!!!!! You can work on my engines anytime!!! It's beautiful!!! If the pitted manifolds bother you coat them with EASTWOOD stainless steel paint. Looks great, lasts a long time. Exhaust manifolds are not cheap. So the heat riser valve is a flap that closes when cold, forcing all the exhaust from that side, up through the intake manifold exhaust crossover, and out the other side. That warms up the intake/carburetor so the fuel atomizes faster and it runs better. Once it warms up it opens and the exhaust goes out each side as normal. Racers delete it but street cars need it. If you leave it in the open position the intake will eventually warm up but in chilly weather it will run rich and crappy until it does. Oil it up and loosen it. If the spring is shot you can replace it. Same for the THERMAC heated air cleaner. It warms up the intake air so the fuel atomizes faster. Take note: once the THERMAC air cleaners were installed, carburetors were jetted to work best with the heated intake air [approx 100°] so delete at your own risk. Since my cars are used all year, I add a thermostatically controlled vacuum switch to shut off the THERMAC after the engine reaches 90°. I only do that in the summertime. In winter I need all the heat I can get. A better idea is the EFE valve used on later engines [Early Fuel Evaporation]. It has a vacuum motor to close the flap and it releases when the engine gets to operating temperature. I added that to our 1968 Buick LaSabre. The heat riser had rusted so badly I added a late model EFE by welding it into the pass side exhaust head pipe. It worked great. If you stumble across a later exhaust manifold with the EFE, you might want it. It has a flat flange that takes an EFE valve between the manifold and the head pipe. But the pipe won't work without the EFE because of the flat flange. That EFE is VERY expensive and hard to find so make sure it is on the manifold. Man, I'm drooling over that engine. Well done!! .