There should be little to no vacuum at the canister at idle, but it doesn't take much of a vacuum leak to mess with idle. Very little will do it. If you study your throttle valve and the way it covers the ports you can see how a little vacuum could make a difference. If your base timing changed, about the only thing I can think of would be a loose distributor bolt, but I'm sure you had it tight to begin with and all. You know you can set timing and idle using a good vacuum gauge, yes? As for timing, a vac gauge will normally have you go over on timing but if you get it as good as it will be and then back off a bit you should be about right.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end