What pistons to increase CR on low compression 340?

There are a few factors other than compression height to achieve a certain compression ratio then add dynamic compression ratio. If using a stock stroke crank that is sound; Deck height, head gasket thickness, head chamber volume, valve head volume, rod length (if incorrectly sized) Piston dome and/or recess volume with compression height all can affect the static compression ratio.

Why do you want to change from 8.5:1 to 10.5:1? Is the engine disassembled? Until you open the motor, you won't know what you have to start with.