540
We just redid one in the club. We pulled the tank out and mounted a new larger tank to the floor with industrial Velcro. Sand the tank down just a bit to get a good gluing. Put the back edge of the tank on the tube. That will also centralize the CG a bit as the fuel runs down. Granted, we did this with a Brison 2.4 gasser for power but the plane is very nicely matched.