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

Absolutely a poor idea on several areas. Chuck them in a lathe if you need to trim them down, but only if they are a solid dome. You can tell from looking at the inside of the piston.

Skip using a .080" spacer and buy Cometic head gaskets in whatever thickness you need them in.

However before you go to all the trouble just sell the domed 340 pistons and buy the set of 360 pistons you need.

Keep it simple for when, or if you need to repair it later down the road.

Tom