I like the wing tank idea except for the construction difficulties. The tank ends up having a lot of surface area for the enclosed volume. One option may be a bladder system. Where you get the bladders I don't know. Or, just plop a regular gallon jug of glow fuel on it's side in a big fat fuselage. The cap would need an improved seal though.

What about the propeller? It seems to me like a prop with more than normal pitch would be desired. This should result in a reasonable pitch speed at 5k-6k rpm. Obviously, the take-off thrust will be down, but the rest of the flight should be more efficient. It also seems like a prop which maximizes full throttle thrust will be way under-propped at half throttle. I'm thinking 8 to 10 inch pitch. I've herd that the propellers used in SAE competition, pylon racing, etc. are jealously kept secrets.

If we want to use a Saito, it seems to me like the 45 or 56 is just about right. Not a novelty in miniaturization like the 30, and more moderate than the 72.

Although the Clark Y seems "old school", it would be very easy to build accurately. Especially if an open-frame structure is used. Any 'foil with a concave surface will be difficult to build unless fully sheeted. Doesn't the Clark Y have a very forgiving stall? That will be important when the pilots start to fade.