To make room for the larger tubs, you must cut into the frame rails. To compensate for the loss of strength, DSE suggests that a person welds 1/8" steel plate back into the missing portion of the frame rail. For this is MUCH thicker metal than what GM actually used on the frame rails themselves.
Here you can see the missing portion of the frame rail.
My plan was to add to DSE's plan and also incorporate GM's design... but BEEF UP both!