It sounds like you get to start your diagnostic procedure all over again; I believe that the only way the EGR valve could cause a rough idle, is if it were open at idle, which it's not supposed to be. When you pulled on the stem you opened the valve. Remove the valve and clean it to verify that there is no carbon holding it part way open. Verify that there is no vacuum at the valve at idle. If it's not stuck part way open and there's no vacuum, you can eliminate that as the cause. As Dan said, a crack between the intake and exhaust could cause your problem, essentially acting as a permanently open EGR. What is your vacuum reading at idle?
Idling problems on these 70's engines can be very difficult to diagnose, especially since the emissions controls complicate the issue even when they're not the cause.