Use a table ban saw to hack off the 340 piston crown .

****, I just throw them in the engine and dykem the decks; belt-sand until flush. If you want the pistons out of the bore, just use an old scrap head gasket as a spacer.
Use a flashlight to make sure the pistons aren't too thin. If you can see light through them, get out the electric glue gun and add some metal where you need it.