I have twin 468 cid motors w/420 Mega Blowers, a little smoke is coming out of the breathers in the valve covers, how do i do a leak down test to see if the rings are going bad? and what should the differance be between the cylinders???

In a nutshell:

Get a leakdown tester and a source of compressed air. The leakdown tester is a block with 2 guages and a pressure valve.
Back off the rockers or run to TDC in a cylinder, attach the leakdown tester.
Set the incoming air for 100 psi and see what the other guage reads. If it's 90 psi or better across the board you are in good shape. If it is lower listen in the oil fill, carb, and exhaust to see if you can hear where the air is escaping.

Oil fill=rings
carb=intake valve
exhaust=exhaust valve

lukiepal

You might want someone to help you hold the crank right at TDC because 10 degree's one way or the other will give you a miss reading.