I've never seen a boxed frame that didn't develop stress cracks, eventually, and my friends on the dirt tracks from Tennessee to California have boxed dozens of them, mostly tri-5 Chevy units. We used to search far and wide for station wagon frames to build our cars with, the ones that ran a factory assembled square tube instead of a C channel frame. If you put an elliptical shape at both ends of the boxing plate, it will delay the inevitable for a little while.
Jerry
