Is gen 6 330hp flywheel the same as gen 6 magnum?
 

I'm building a new motor using a gen 6 mag as the core.
I understand it is externally balanced.
Will I be able to use the flywheel from a gen 6 330hp engine?

I don't see why not. All 454's are externally balanced from the factory to my knowledge.

One is steel crank, heavier rods, forged pistons. One is cast crank, lighter rods, cast pistons. There are 3 diff schools of thought. One sez different flywheels for forged cranks, one sez all take same flywheel. One sez same flywheel balanced differently. One thing for sure. MK4 flywheel is not the same because there is a balance weight on the back end of the 2 piece seal crank.

Are you having motor balanced? If so, flywheel-balancer go to shop with engine and flywheel will absolutely work. If you are assembling stock parts yourself and trusting GM to having the parts balanced acceptably ahead of time there will be more people weighing in here. This is not the best place to get that question answered...It's the ONLY place!

I am strongly considering having the entire rotating assembly balanced. Is that expensive?

Getting it balanced will be more expensive than buying a new correct flywheel. About 150 or so at Jegs. Also, if your pistons are already mounted on the rods they will have to be taken off to balance them and also to get their weights for the balance calculation. A new balance is nice, but these generally are not 6000+ rpm engines.