Right you are Sam - sistering will add strength,but alone, won't it eventually flex and bow with all of the lateral torsional twists from the engine? Your idea of a X brace in the middle of the frame would help a lot, there is just to much open area there.It looks like the spring perch's are "sistered" to the front of the frame rails,capping the ends will help in the twisting and strength. But before any thing can be fixed, the bow has to be taken out, then contend with the front end.