Before you hard mount your servos and gear you can tape things in place on the outside of the plane to find the mounting location to get your CG. I do that to all my planes. At the moment I'm building a Cub for a friend. while in the bones I have used tape to hold things in position to find the CG. Using an OS .91fs in a 40 size plane so it was nose heavy from the get go. I ended up mounting the two servos for the elevators and one servo for the rudder behind the cockpit and made up hatches to get them in and out. Not completed yet but it is CGed or close to it. This is going to be a float plane so everything has to be mounted inside the fuse. As big as this fuse is it's getting crowded from the mid section back!!