One more note, If they agree to put a backer plate on the frame at the outside of the DC mount, have the clearance the outside backer plate like this:
Their backer will have holes where mine has studs. If they don't want to clearance it - and this should be ok too - make sure they clamp the DC mount to the frame top/top bottom before fitting up and drilling the backer holes. The bottom of the backer will be slightly higher than the bottom of the frame, but the DC mount will be flush to the bottom of the frame.
I don't think the inside plate needs this for your setup because the inside is welded on and already at 90 degrees.
