i can't see how it would reduce much, if any flex on the lower rails considering that the engine mount bolts tie both sides of the rails together anyway. If the motor is a snug fit, as it should be, between the mounting lugs, that in itself reduces any flex down the bottom of the frame. Re welding the old welds is the best way to reduce flex in my opinion, and also reduce the risk of frame failures at fractured weld seams.