I heard back from Sling Technical, and I do, indeed have the old MGU mounting bracket. They also cleared up the CF-PLT-005 Side Rail Joining Plate confusion. The plate goes on the inside of the assembly, and the cut corner faces the rear. Three of the holes in the side rail were too small for the rivet size in the plate, but Technical said as long as there was 2x the edge distance, it would be fine to enlarge those three holes. I emailed Scott in Torrance asking for the missing AN3 Anchor Nut Plates, rivets for them, and a quote for the ELT bracket. I trial-fit the side rail assemblies, and now that the CF-PLT-005 confusion is cleared up, everything went together perfectly. The CF-PLT-005 Joining Plate needs to be bent ever so slightly to conform to the curve of the side rails, which was easily done. I touched up the missing primer on the floor skins.
When I was looking at Midwest's web site, I noticed one of the options they offer is a laser-etched and powder coated throttle quadrant. You send them your original, and they send you a very nice one in return! I'll be sending mine out in a few days so hat I'll have it when I need it.