When you are working on an engine as old as the 327 (last made in about 1968), you have no idea who owned it before and what modifications they made to the engine. One of the problems that can arise from changing manifolds is that when you get the engine back together, you could have a vacuum leak. This can occur as easily as unknowingly letting one of the gaskets slip as you are installing the manifold or it could be as hard to find as a set of heads that were milled by a machine shop many years ago and now won't line up exactly with your brand new manifold. Maybe the heads had been milled and the old manifold had been milled to match the heads, you see, you just never know what has been done to the engine in the past. I'm telling you all this because you need to be aware that problems can arise and you wouldn't know anything about it if I didn't tell you. If you get the engine back together with a vacuum leak, no amount of adjusting or tampering with it will fix it. You just have to tear the motor back apart and fix it properly. Here's a paper I wrote on this subject. I think you should read this thoroughly and understand it thoroughly before you ever lift another wrench.
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