I'm putting on a pre-smog, cast iron intake on my 350 (Cam & headers). My question is reguarding the exhaust blocking plates that come with the gasket set. Do I use 1, both or none, as I'm considering? My thoughts were that by blocking both sides, I could reduce heat to the manifold/carb. What are the thoughts out there?
I'm using the cast one to keep the cost down ( bought this one for $5.00 at the bone yard), and I figure the extra pounds won't matter if I lay off the Pizza!

Intended use of your car is the most important question here. This exhaust heat crossover passage was used with a heat riser valve on the exhaust. This was intended to shorten warm-up time of the engine in colder climates. If you are going to drive your car daily, this feature is helpful. For summertime cruising only, block both holes. Depending on the year of the engine, one hole was partly blocked with a special gasket and the other left open or both holes were left open. My '73 350 engine has one hole partly restricted but since I don't have a heat riser valve and I live in Texas, I don't worry too much about long warm up time. I should have blocked both of them off but so far I haven't experienced problems with my setup like this, I have a 5/16" thick gasket under my 2-barrel carb that insulates heat quite well.

Thanks Iceman. Thats the input I was looking for. Although I live in Michigan, this project is for summer cruise'n only.

Less heat in the intake manifold means more power. You might want to reconsider a modern aluminum manifold. The aluminum intake manifold is lighter but the main advantage is increased airflow; you could be giving up as much as 20-30 hp using that cast iron manifold anchor. Look around you should be able to pick up a used manifold fairly reasonably. Even the cost concious factory used aluminum manifolds on there max effort engines from as far back as the '60s. Besides a new cam with 5500 rpm potential won't do you much good if the manifold won't let the engine get much pass 4000 rpm.