I worked as a engine tester at the Tonn Chevy Plant when the 400's when being built. We would balance those engines in the test stand. We almost never rejected a engine because of the flexplate being out of balance. All the balancing was done with oz pins driven into the balancer. We didn't have a welder to add wt or change the wt to those flexplates. Where they balanced to race tolerances??. Certainly not.
I have read through this thread. My suggestion would for this O/P to take the block to a machine shop to have it checked to see if it needs to be line bored/honed. And as stated have the rods checked as well. Then have the block hot tanked,rifle brush cleaned,new soft plugs and cam bearings.
My Howards 383 crank states clearly on the side of the box any alterations to the crank,you then own it.