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

SLOPAR72 said
You need .187 or so for material thickness on the top of the piston if I remember correctly. This was a set of TRW's but it's been 25 years.

Make a simple fixture to keep it consistent and way to scale your pistons. It won't win awards from most but I did it and that motor is probably still running today lol.....
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Thx for the motivation. I decided to just use the 3.79" crank with TRW L2405F pistons and thin head gasket .028", and speed Master Aluminum 65cc closed chamber heads...uneducated guess will put me @ a hopeful 9.5- 9.8 cr...I am sure some expert out there will correct me on this statement.