I learned 2 things over the years with engines, only trust yourself and smart buddy with own eyes, and doube check all machinework!

Thinking the wear marks on pistons might be from crap getting wedged between, only 2 had them marks...seems to be internal balance..

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usually when they internally ballance the engine they add bob weight,,i like to use bryant and winberg light weight cranks and you will see a hole driller through the counter weight for internal ballancing..i dont ever remember seeing a 8'' ballancer with out a counter weight.. when you get it off look and see if theirs caster weight on one side and also look at the flywheel..over 600 hp i like to use a billit flywheen not cast..

here is the crank out of my 632,,you can see the weight in the crank..you having a forged crank and 6.385 rods its soppose to be internally balanced..

According to pbm it's internally balanced, I don't see any balancing holes in damper either.

BALANCING HOLES IN BALANCER???? The balancer is most likely; neutral ( no counter-weight )

On my fords the counterweight one side, holes or dimples on other. I will send pic later.

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On a Chevy if it's externally balanced it will have an added weight on the balancer and another one on the flywheel.

Not always. Some 1 piece rear seal motors the external balance on the flywheel is done by drilling out material - not by adding material.

This has caused many fits getting flywheels for motors - especially the small blocks when stroking. Once the info is known, the fit stops. LOL. Thus why I'm sharing this.