put your V tail upside down on your workbench. Put the fusealage on and level it on your work bench. So now you have both the v and the fuse upsidedown and leveled. Put a straight piece of wood accross the rear of the fuse. Measure up from the bench to the wood that it is the same on both sides and glue your stab . You'll find this is the easiest method of doing it.
Dennis