I pulled my old engine to get ready to insatll the new 383.
In the past, I had vibration at approx 1500 RPM and also 1t 3-3500RPM.
I pulled the flywheel and noticed that the alignment dowel was missing.
That puts two possible causes for the vibration.
1. The flywheel was not mounted in the 'dowel' location, meaning that the balanced flywheel was off location to the flywheel.
2. The dowel was missing and this would mean that some mass was missing causing and inbalance.
Here is a photo of the end of the crankshaft and the missing dowel.