Ken - I put my two elevator servos underneath the stab but just behind the leading edge of stab had to stack them one with arm down the other with arm up.
As far as Gas Tank with gas you do not need that large of a tank, gas uses half the volume of glow per minute roughly. If using a 4 stroke it is even less then that.
My rudder servo I made it a pull pull with Kevlar cables. Servo in center at rear of opening. There is plenty of room for ignition and batteries in the nose.
Not too many scratch builders anymore, and this is a little tougher then most. There was a fellow that would cut your ribs, bulkheads and such. Think out your main landing gear first, it sets a lot of the ribs, I would strongly look at carbon fibre. The plywood as it shows gets quite heavy. Make sure axles are ahead of leading edge, it likes to nose over. Gary