Kicker Advice Needed

[i4110peter]

I want to buy a 9.9 4 stroke to use as a kicker on my V20. It is an I/O version. I see different shaft lengths available and don't know what I need. I would like to have the kicker attached to the I/O for steering. What measurements should I make to determine what shaft length is appropriate. Any suggestions will be appreciated. Spring salmon is right around the corner and I want to be ready.