Engine shakes BADLY!!

Engine mounts do not cause an engine to shake. Although "something hitting" like a header beating against a T bar can certainly transmit vibration, etc.

Might be time to go back to basics

Has the carb got a problem, IE do both left and right side idle circuits work?

Do the idle screws have "good" effect and is there a definite "peak" on the adjustments?

Pull the carb off........are the throttle blades properly located in the idle transfer slots?

What is your initial timing and how did you decide on it?

Have you actually checked continuity on plug wires?

Don a dynamic cylinder balance test? That is, short out cylinders and compare RPM drop?

Run a cylinder leak down test?

Compression test?

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Have you for absolute certain eliminated a balance problem? (I don't know what you have for slugs, crank, and rods..........so.......... ......... or converter, weights, flex, or balancer...................).