Right, so imma edit the heck out of this post because it wasnt nearly as complete as i wanted it.
To start with, a plane at the 40' span is definitely possible, but you're probably going to have a LOT of issues keeping the weight down. Just as an example, I'm in the process of designing an 18' model of a b-36 right now. This plane has everything going for it, a simple fuselage, an airfoil thats super thick to the point i can minimize spar material. I'm targeting an AUW at or under 55lbs, and i should be close but make it. Because when you break that down, its about 10lbs per wing, about 10 for the fuselage, add 5 for tailfeathers, and the rest is going to be motors (6lbs), batteries (8lbs), props, payload, etc. So if you want to do something 40' i would say the option to best stay as far away from the AMA is to stay away from a full-sized copy, i.e. make a Cessna 172. Either do some tip join aircraft or something like the Helios.
And to be honest, i'd strongly lean towards helios for one reason: wing thickness. I'm sure you saw how the Flex 9...well, flexed
Guy's flying this thing hard as he can, and this plane is doing all sorts of squirrely stuff. Flex doesn't just mean wing dihedral, it also can mean asymmetrical angle of attack, which means you can wind up with uncontrolled control movement, and, in extremes, areas where parts of the wing is in a stall where others are not. This is why i wouldn't just go gluing planes together, its all about leverage. A plane is , ideally, designed to handle the stresses that one aircraft can exert, including multiple G's from high bank angle, turbulence, etc. The problem is when you tape a bunch of planes together, you add new factors, including wing spars being exposed to forces at a much greater distance, like a lever. A small force applied to a wingtip can wind up being a massive load on the center aircraft.
On top of all this, lets think logistics here. If you want to do a 40' airplane, you need to get it to point a and point b. If you think that transport and logistics is inconsequencial, be reminded you are endeavoring to build something with a larger wingspan than a Piper Aztec twin-engine aircraft! How are you going to get the aircraft from your house to the airfield? How are you going to get the plane into the air, does it have wheels, or do you have friends that can help you throw it? Once its in the air, how do you land?
If I was doing this, i'd probably do something like the helios. Make a big, 2 or 3' long wing, and give it an appropriately thick airfoil. My inner b-36 project says NACA 63-422 or 420 would be a good candidate, but i'd go talk to some people with experience in flying plank designs. You need a thick airfoil so you can minimize weight for structure and also keep the wing more rigid easily. Then i'd have 2 or 3 fuselages you can attach to the underside that have your powerplants, that or just scatter tiny powerplants all over. If you want to make the fuselages with an elevator and all then i would strongly recommend oversizing the elevator, or better, full flying elevators, and place far back using CF booms alongside rudders sized for the task.