UnBalanced SBM Engine build

The one you threw together had cast stock replacement pistons (I don'y know of any cast performance ones...). They would work fine without balancing. "Fine" being defined as "as good as the factory did" which in terms of real performance which was terrible but runs ok in stock form.
The new engine needs to be balanced. If this is a short term fix - maybe find another place-holder or just anticipate holding the paperwork and when you sell it - it's a selling point.