Big Block Billy

454 mark 4 for a certainty, I know the number by heart
....456 N is Neutral balance
The N is for Nodular Iron.....
Spend the money and make sure everything is balanced together. Look for pins in the flywheel and dampener that were added. Usually only high performance motors had them from the factory indicating a closer balance job as opposed to a "ballpark" or "Detroit" balance job
There are also 2 different flywheels and 2 different dampeners for Gen5/6 depending on cast or forged crank.
the dampeners will fit from a mark 4 to a Gen 5/6 but the is not advised. Usually they can be balanced with the assy.
Just saw a Gen5 Cast rotating assy that was out over 100 grams with a fully weighted "pinned" balancer. Ended up being the wrong balancer. It's good to find this out on the balancer. If it wasn't on the balancer, After it blows up it may never be found out why ! Or even worse, a second or third blown up motor !
I had a big blowout with someone on ebay selling a crank that told me I was wrong. If I'm wrong GM is wrong Mercury marine is wrong and I guess the parts don't exist and as long as it bolts together it works because without experience it's going to blow up anyway ! The problem with this stuff is it's been whored up by too many people or parts switched around etc. Hope this helps, BIG BLOCK BILLY