Gun fit for side by side and O/U are almost always different. You put your finger on the main difference. O/U recoil is rearward and upward while Side by Side also has a horizontal portion of recoil. Same amount of recoil spread over three vectors will caus less impact to the cheek area. If you add a little cast off to your O/U you can reduce the cheek impact. Also if you have a fat cheek area, this will allow you to get your eye centered on the bead rib area without rolling your face over the comb.
If you are going to get fit for a stock do not expect one measurement to translate for both types of guns. A few are that lucky but not many.
