A 91 should make up for the drag specially an O.S 91fx what servo's are you going to use. I'm not sure if standard ones are enough you will have plenty of space inside. My extra is 60 to 90 size and comes with a 8 oz tank and am going to have to modify it cause there is no space really hate that.
Are you going to build a box section to seat the wing into and what are you going to use also what size of stringers will you use.