head issues and how much to cut off the intake.

I purchased a set of heads that seemed to have had no machine work done to them. I had them reserface, valve job, and new valves and spring put on. When I got them back from the machine shop I decided to check the cc on the head and found it to be 68-69 most where 69. Appears that theses heads had be cut before I purchased them. Now i'm tryin to figure out how much i need to cut off my intake manifold. Could anyone tell me how to measure how mucht to cut off the intake.