If you want to be the guinea pig, you could get the neutral balance flex plate from 440Source.com, then cut out a notch to remove weight to make it Magnum balanced.
I have one of the B&M flex plates that is Magnum balanced, and I could mail you a paper template to cut the notch. B&M no longer offers their Magnum weighted flex plate, but the one sold at 440Source is cheap (NON SFI approved for racing) and says it has the large pattern that uses the smaller 5/16" bolts like your converter. I have not tested this method yet, so that's why I say you'd be the guinea pig.
Solid Flexplate - 6 bolt - Neutral balance. New-440 Source
