You can remove .125 off a passenger car block. You can remove about .200 from an X block. An R block or Ritter can take .300 plus off.
I’m begging you to NOT buy a KB piston for anything. The ring package is just junk. Saving money on those pistons is a bad way to save.
A 5/64 ring package is obsolete. If they would just use a 1/16 ring pack it would be different. But they will not do it.
Really they should all have an .0394 ring package (1 millimeter) but that would be asking them to move from the horse and buggy era to the ballistic missile era in one step. It just won’t happen.
There is nothing wrong with those pistons other than that garbage ring pack.
At any rate, the block should be machined because it’s not flat, unless it’s already been done.
I’m not a fan of milling the hell out of a head for compression.