[Cheesehead]

If your engine starts shaking it could also mean that your ignition is too advanced at that rpm. Somewhere in your post I read 18 degrees. That seems much to me. I have also a 351W stroked to 427. My engine likes 14 degrees idle advance. Total advance is 33 degrees at 2500 rpm.
From 850 rpm upwards it starts advancing. Usually this number is about 100 to 150 rpm above idle. My idle is 700 rpm.