I played around with it for a while last night - moving studs, trying
to tighten different ways, etc. At one point I got the faceplate
pulled fairly tight against the mounting plate, but it still wouldn't
run true. I finally clamped the faceplate to my mill table and ran a
dial indicator across the different surfaces. While all of the
surfaces are flat, they don't appear to be parallel to each other. The
bottom line is that this faceplate is a piece of junk and will need to
be replaced.

Thanks to everyone for your input.

Nick