360 engine vibration

OK, on the compressions.... so less likely in the cylinders or valvetrain. A really bad lifter that would make it feel like a cylinder is dropped would more than likely show a different compression reading.

I would definitely take off the valve covers and check the lift and inspect all the valve train components. Also, after a brief warm-up, push down on each rocker at the pushrod end when its respective valve is closed; any softness will indicate a leaky lifter. To check them when each valve are closed, you will have to rotate the engine a bit at a time.

Strictly speaking you need to check lift at the lifter body to make sure that the cam lobes are all good. If a lifter is collapsed for example, then the lift there will be low but the lobe will be good.

Do you have an ohmeter? I would check the end-to-end resistance in each plug wire to see if they are consistent.