BB,
Think of the rails as a suspension spring in a car, be it a leaf or coil, if you stiffen up that spring, the axle loads transfer to the spring mounts, what will happen to the mounts?
Changing the flexible nature of the rails will only move your problem elsewhere. I would imagine repairing a side plate with a large crack or hole in it would be very costly and difficult.
Boeing,
Comparing the 1970 B&S with the 2015 B&S they both still wobble around. Getting the right engine speed is the key.
