[Arran]

Buzz;
  A lot of 1936-38 RCA sets used a similar chassis isolation system to your Stromberg Carlson, fortunately most of the ones I have found are still usable. The closest thing that I have seen to that "T" nut and washer was something a representative of a hardware supply company demonstrated at a car show, it was an insert that you could use in sheet metal to hold a machine screw. I don't know what the proper name of these inserts is but the device he used was something like a pop riveter that would expand part of it so it would not come out. If I remember I will go down to their store and ask them about them.
Regards
Arran

P.S The cabinet top is warping because there is veneer on the bottom side, which is also still sealed, but none on the top. So the boards that make up the cabinet top can absorb moisture and dry out faster on the side where the veneer was removed. When you re-veneer the top and seal it again it should stop doing that, but to flatten it out just wipe it down with a damp rag and clamp it first.