Using the TCDS data you provided, I tried a few different engine positions and I am still of the opinion that I will be required to move my O-320 forward about 5 inches
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As I look at your numbers, I'm not sure you put the prop in the right spot.

On the known configuration, the prop is forward of the flange so the moment arm would be greater than 56".....( I would guess 59" or so.

Same for the 0-320; the prop CG will be in front of the flange.

It looks like you are on the right track.

Again, I don't know for certain whether the TCDS that I posted are the latest.

I'm familiar with what you are trying to do as I went through a similar exercise when I was thinking about putting an 0-320 on my LSA in place of the 0-200.

Good luck!

Brad