I recently finished a complete rebuild and engine swap on a 2 bolt main 400. The engine is now installed and fired right up with no issues at all. The engine is outfitted with the original harmonic balancer specific to the 400 and a new externally balanced flywheel (the motor originally had a flex plate for an automatic transmission). Despite running perfectly the engine has a massive vibration from just above idle and up. The vibration is so strong I'm concerned I may crack the bell housing or other. Any thoughts on what may be causing the problem... old balancer faulty flywheel???