Hi Jack: I just plucked the figure from a table I had on hand of tube section properties. The section modulus is easy to calculate, and once you have it, just divide the moment in pound inches by the section modulus to find the bending stress. I would try to find a piece of 2024 in preference to 6061, since it is quite a common alloy, and is about 75% stronger. I use 6061 only where extensive forming is needed, or welding. 2024 is hard to weld, while 6061 is a breeze. You might try this website for section modulus - (Sx):
http://www.onlineengineer.ca/circle3.htm
It asks for the dimensions in millimeters, but if you plug in inches instead, it should work just fine, and the section modulus it calculates will be in inches cubed, not mm cubed. I recently helped a local modeler make the same calculation for a big AT-6, and the recommended wing joiner tube was a bit shy of strength, when I calculated the moment that the wing would develop at maximum expected speed for snap roll entry. Incidentally, I get about 38 pounds lift per square foot at 100 mph, using safe lift coefficient of 1.5, which should be about right for the scale wing section. When calculating the moment developed by the wing, you can usually subtract around 20% to allow for the negative moment developed by the weight of the wing, so using 25 pounds per square foot to figure the actual moment on the tube might not be too far off.