Actually, the BB spindle won't flex much at all. The torque from the right crank arm pretty much gets transferred directly from the crank spider / chainring to the chain. The torque from the left crank arm is transferred by torsion through the spindle to the right crank arm. Bending deformation of the spindle is negligible, since the bearings are very close to the crank arms. An external BB with a large diameter hollow spindle will be a bit stiffer, but the difference is very small when compared with frame and wheel lateral flexibility. If you are really concerned about crank stiffness, get a true track crankset with a 144mm bcd and 1/8" chainring.