"Misses" at Idle, but not at cruise RPM???

[Tomaso]

I have a 72 GMC K2500 with a 25000 mile or so Mr. Goodwrench crate engine. I just replaced the tired old original carb,as well as the original intake manifold with Edlebrock "performance." parts. Also put in a new EFI distrbutor and new plugs and wires. Truck runs great now! Only problem is that at idle (800 RPM) the engine seems to have a big miss that you can feel in your seat! As soon as speed increases, the engine smooths out nicely. Since I already replaced the ignition parts, I'm now guessing valves or maybe a vacuum leak. Anybody wanna take a shot at this?
Thanks in advance for the help.