Grinding or machining of spacers to fit a particular oddly shaped truck bed flat is for sure not 'reengineering' the design. As far as the orientation of that pad - either way is correct, depending on the width of the bed rib channel per their instruction sheet, pg 2 (
Companion? - 5th Wheel Hitch | Single Point Attachment | THE B&W ADVANTAGE).
"Locate the two plastic foot pads. Each pad attaches
to the base through a slot at the end of the base leg.
Before attachment, check the width of your bed rib by
setting the wide face the plastic foot in a bed rib, see
figure A1. If the plastic foot does not reach to the
bottom of the bed rib trough, turn the plastic foot over,
see figure A2 & A3. Locate four 1/4" machine screws,
1/4" flat washers, 1/4" lock washers and 1/4" finish
nuts. Insert the machine screw up through the bottom
of the foot pad through the slot and attach it with a
1/4" flat washer, 1/4" lock washer and a 1/4" finish nut,
see figure A3. Do not tighten hardware at this time, so
that the plastic foot is able to move back forth ........."
Adding any gusset or reinforcing would be reengineering the base and yes, B&W should take a look at the OPs mode of failure rather the us here on a forum second guessing the cause - and hopefully he has given them plenty of info so that they can take a look at that design.