For your sx3 with cantilever you need to make sure that you order the model that comes with picatinny mount. For your dads M2 you need to decide what base you want to use. If you are installing a Weaver type then he will also need the picatinny mount. If he is going with something other, like a Burris 336 base then he needs to order the 336 base and a FF3 with or without Weaver style mount as the Weaver style mount is removeable. The FF3 mounts directly on the 336 base. If he does decide to go with the 336, then he needs to wait until you get the base so that the gunsmith, if he is not familiar with the 336, will need it for a template to get receiver drilled/tapped for correct hole spacing.
Ask more questions if you need any other information. I have mounted about 6, no expert, but have mounted them in several configurations.
Ridge Rooster