Shake rattle and roll

Update:
So I found an early style magnum balancer in decent shape and installed it but I am still having the same engine "throbbing" at idle and cruising. It is definatley more prevalent when the engine is warmed up. The engine always stays nice and cold. It is not a full rebuild I found a rolled truck with 102k on the odometer. I tested the compression when I first picked it up a year ago and it was running good numbers. The motor was visibly clean when i pulled it apart almost no gunk or carbon. No scoring on the main bearings ot other signs of damage. I have ran a vacuum gauge it sits perfectly steady at 21. Fuel pressure is steady at 7 psi. Good oil pressure. Carburetor is a 1405 edelbrock that ran flawlessly on the old motor. I double checked the plug gap. The new plugs have maybe 4k miles and all look like theyre brand new no indicators there. Played with the idle on carb. Used starting fluid to check for vacuum leaks. Stock magnum cam, the timing was set at 14 degrees. I brought it back to 6 which is stock for a 78 smog era cordoba la 360. Still cant figure this one out. I know the exhaust is a bit constricted (la manifolds with a stock cat) but its not sluggish. Just has a surging vibration driving me crazy. You can feel it in the steering wheel once its warm.