The block is a piece of a cast iron that does not rotate so it has nothing to do with the rotating parts inside of it
My message is you now have the ability to have your new parts balanced either externally or internally, the differences are in the harmonic balancer and torque converters you have to use, if you want to be able to swap converters later it is easier and safer to use a internally balance converter now instead of having to have another custom externally balanced converter made to fit your motor later
You can use a standard neutral balanced harmonic balancer with a aftermarket externally balanced flexplate to use with internal balanced converters also , you have several different choices to be made now