Magnum mount issue?

I gotta go with the fish guy here. Your problem is engine balance. You can go with a Fluiddampner, I did, but you'll have problems with your crank pulley. (Yet another call to March).
Short story, we put a healthy rat motor into a Chevelle with a 5 speed. Got huge vibes around two grand. Pulled it apart and found we had the wrong flywheel. (502's are externally balanced, we had reused the 396 flywheel, internal balanced). As for the vibes being worse on decel, it simple physics, objects at rest want to stay at rest, objects in motion want to stay in motion.

Revhendo