How about timeing chain jumping a tooth? Sticky valve? Debrise not allowing it to close? Carb dirty? Crossed ignition wires? Broken tranny mount/rubber ripped/torn?
You claim it purs. Then you say it started to shake but still ran smooth.
It is hard to guess what the problem may be on line and I'm just offering a thought on it. OK, stabs in the dark. But it's dark were I'm at. (LOL :offtheai:)
Check it anyway. Nothing to loose except a few min.'s time.
Also Adams suggestion on Magnum heads is very good if the X,J,U heads are not found. On a 3-8-teen, or any small block (In general) there likly to perform better due to the closed and reduced cc head chamber.
