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

6PakBee said
But I know for a fact that you can take a wood router with a milling bit and cut some surface from a piston dome with the piston in the engine.

I know for a fact, really have you done this......no safety issues here

6PakBee said
A safety hazard is a safety hazard. Now you two can yuk it up all you want but the fact remains that is not only NOT a recommended method but an unsafe one. Period.

So a wood router and a metal milling bit is OK, I don't know where you're from but you need to stay there.