I think we need some more information:

--did this vibration happen from the first time you started the rebuilt engine or is it a new phenomena?

--did you change anything other than the cam? New pistons?

--does it vibrate at idle in neutral?

It seems to me that vibration at idle in neutral can be caused by three things: Bad motor mounts; poor ignition; something out of balance in the engine. If you have checked the mounts and are confidence you are getting good ignition in all cylinders, then something has to be out of whack in the engine. A bad timing gear might cause this, but you would likely hear noise. A badly out of balance flywheel could certainly be the culprit. Did you have the flywheel resurfaced? Are you sure you kept the rods and pistons in the right order? The only way to address these things is to pull the engine again, I'm afraid.

I am no expert, though; hopefully someone who knows more will weigh in.