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

Yes, I believe your correct, but now I know some poor fella / youngster out there who has an old set domed forged piston laying around and wants to build his engine with no money and use what he has will go and borrow his neighbors table ban saw and enjoy doing it.

Sell the pistons you have and buy some 360 pistons. No reason to reinvent the wheel now a days with all the choices that we have for parts.