At the APVM ( Monbtreal Windsurfing association, we found that the iDO did not have to be scrwed on the sides as log as the sail was a kid's rig less then 2.2 meters. As the iDo has 2 sets of bolts at the bottom of the base, it will give you a good hold of the rig.
For the stability of the board with the iDO, I would recommand the 175 lt. The more buoyancy you will have for your board, the more
Have a nice day, and BON VENT ( Good Wind ). Eric Gouin Président APVM