We use a sonic test to determine how much wall is left.
It is ideal to remove the minimum diameter of metal from your bores but that's not the only consideration. I had my Y-block bored +.060" because that was the closest size piston that was available at the time.
So the question is, who will stock all pistons in all sizes and how many sets will they keep on hand? The pistons are cast in batches, then the machining is set up in 'production runs'. They make thousands at a time. When a specific size is depleted, the next run may take a while because NONE of this stock is paid for until it is actually sold.
Alright, having said all that hot air, let's get more practical. If you're already +.050" you're at the end of your rope. Ten thousandths more is only FIVE thousandths per side. That's twice the thickness of a human hair; hardly enough to 'clean up' deep scratches.
I would look for a common 390 short block, either from a pickup truck or a car. If it needs a +.030" bore, so be it. There's plenty of casting left. - Dave