a 2 x 2 x 7 tank seems to me to be far too long a tank for the 2 x 2 x-sectional area, and is probably only used in large models which at speed and aerobatic has centrifugal force keeping the fuel in reach of the clunk at the rear of the tank. If you watch these birds landing more often than not the approach is slightly nose up at the slower speeds and this also helps with fuel feeding from the rear of tank.

At the end of a training flight- usually nose down attitude for landing - as the nose goes down then fuel will run down the near empty tank away from the clunk which is not meant to bend back on itself, and certainly not that length,and therefore the engine will be starved of fuel and stops.

exhaust pressure will be maintained on the tank regardless of size once pressure is built up as exhaust is normally more than adequate to keep the pressure up as it replaces the fuel draw off which is minimal compared to exhaust and pressure produced and is relatively constant for each engine irrespective of tank size.

shortening the fuel pick up to get the clunk 1/2 way back up the tank simply hastens the time when the clunk wont see fuel in the tank even if there is fuel - and this will vary according to the attitude of the plane.

If there was plenty of room I would tilt the tank back 20/30 degrees to try and keep the fuel at the rear of the tank and next to the clunk which should just about cover most flying attitudes for normal training flying including landing.

Failing that, I would get a smaller tank as suggested above or time your flights to have more fuel left.

I think I get more time out of my LA 40 , but then I throttle back after take off and fly at a speed more compatable with both the planes natural speed and my own innate flying skills.
John