SAKO300, the first thing you will have to do if you want to mount on top of the rib is, level the barrels in a mill, and mill the two places where the mounts are to go level with the barrel pair's bore, and with each other! The ribs are rarely, if ever that level. That may not sound bad, but when you mount the rings they will not line up if both bases are not exactly the same!

Take a set of rings clamped down on the bases, and stand them on top of the rib, and look through the rings, and I think you will see they do not line up. One will tilt to one side, and the other to the other side, or both to the same side, but by different amounts!
There is nothing wrong with mounting a scope on a double rifle, but it should be done properly.

Good luck!