Chevrolet Repair: vacuum switch, 1975 chevy malibu, chevy malibu

Question
I may have asked this before. If so, I apologize. I got these automated emails saying my questions had been answered before but I only received one.

Trying to find the name of a part that sits on top of the exhaust manifold of a 1975 Chevy Malibu 350hp engine (2bbl) and has a line going to the carb. The part description is what we need, so we can try to find one. We have been told it is a "thermal riser" or "thermal vacuum switch". No parts house recognizes the name.

We also want to optimize the fuel consumption. If this question has already been answered, can you tell me where to find it?

Thanks

Answer
Theres a vacuum valve mounted to the exhaust manifold via a "A frame" style bracket, pass. side, rear. It's linked to the heat riser valve via a rod, about 12 to 14 inchs long. This would be the heat riser diaphram assembly, probably only available through GM. For best MPG, engine must be rebuilt with more compression, cam designed for weight and gas mileage. If engine is in pristine condition, follow tune specs, and add 2-4 degrees more timing.