Want to bolt the http://professional-products.com/har...armonic-damper to a stock 455.

I know that all Olds motors are externally balanced so only what i have to do on the balancer before install is ...nothing

because external balance : the weight stays on
and Internal : the weight comes off

is this correct ?

 

Kind of. But you're taking for granted that any aftermarket balancer has the same amount of external weight and in the same place as the stocker. That's a lot to assume.