Where to start?...A vibration could be anything and volumes can be written on the subject. That said did it always vibrate from the time you built the engine? What flywheel? Vibration dampener? Does it vibrate out of gear, in gear, both? From my experience with a similar vibration problem on my F250 2wd, 390 +.030" mild hyd cam, single 600 Holley on a Streetmaster manifold it also had a shake at about the same rpm as yours. After changing the Street master for a iron "S" manifold, no other changes, even the same 600 carb the shake/shimmy was gone. All I could think of with this swap was the S/M had a idle/low speed fuel distribution problem. The "S" manifold is a dual plane, small runner high velocity manifold. I did not need to save the 50 or so pounds using an aluminum manifold on a 4800 lb truck. An added benefit of the S manifold was that my fuel mileage increased about 1.5-2 mpg, better low end torque even increased midrange torque somewhat. IMO a win/win situation all the way around plus it freed up a aluminum manifold for something more worthy and beneficial.
