Check your distributor cap for a small crack or moisture and make sure that your plug wires aren't cross firing due to improper routing. Either of these could cause a small miss. The vac ought to be stable at idle on a stock engine but I would drive it and see what happens if it doesn't go away then check valve adjustment. Always check the simple things first. Your small miss could be causing the unstable vac at idle.
Been there, Done that, Hope to live long enough to do it again.
Big Bill
I.I.# 4698
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