Have the rotating assembly balanced, trying to find the correct external balance flywheel USED, is going to be next to impossible. the balance job from the factory wasn't that good to begin with , do yourself a favor and have it balanced using the forged crank.

Next dumb observation ... rotating assembly KIT ??? be careful , 400's were low compression from the factory there are very few pistons available that aren't .100 or more in the hole, any would be from companies like ross, diamond, etc ...

Sounds like you are trying to do this on the cheap, you may be disappointed with the outcome.