Good call BHP, can't he get a piston from WSM that will be 5mm off set but available on 66.5? I've never carried stock cyls past 66 with out going to a big bore 68mm^ Sleeve, so the blaster piston thing I'm not up on all the blaster piston stuff.
Is running Minimal base gasket and decking the cylinder an option? After all, the top end in question is at the end of it's life.