Can’t most CX bikes fit 40mm tyres anyway? There is plent of clearance on mine to get another few mm in.
I’m still trying to work out if I’m free to enter but If I do I think it would be a difficult choice as to which bike. For the 101km I think my body could take the beating of a CX bike but over 100 miles I don’t know.
This from having done a 100km ride up there this year already and done the cross border/newcastleton ride on a cx bikes as well as a ful suss. There is a lot of gravel but it isn’t small gravel it is really quite rough. MY arms and back were as tired as my legs on the old K100 course.