[Mike_The_Grad]

Try hooking up a vacuum gauge to a direct manifold vacuum port, they only cost about $25-$30 @ harbor freight and come in handy for these older engines. They can be a little tricky to read sometimes, but there is a ton of info on here or Google about reading the results. Ideally, a constant steady needle in the 17-22" range is what a engine with no vacuum leaks looks like. Obviously if you have a large cam like the Comp Cam 268h, your reading would be lower than those values. For that you would have to check what Comp Cams lists the vacuum this cam produces. Idle mixture screw adjustment won't tell you if you have a vacuum leak. You should be able to kill your engine when you "lightly seat" one of the mixture screws at idle, and the engine should definitely die when you seat both of the screws at idle. That tells me you may have multiple issues causing your high temp issues.

I say this because I had this exact thing happening with my freshly rebuilt SBC. My timing curve was off and my vacuum advance canister had a ruptured diapraghm creating a vacuum leak. It was hard to figure out the diapraghm was ruptured because when you set engine timing you plug off the port that supplies vacuum to the vacuum advance and plug it back in after the timing is set. When you have the shop check your timing, ask them to hook up a vacuum pump to the vacuum canister and verify that the diapraghm is holding a vacuum.