I think with a combo of filling low spots and planing high spots this can work out fine. You could always veneer sides and front and paint back or use some textured coat and paint flat black. Take that straight edge and place it corner to corner and here and there to determine highs and lows. Mark the high areas with pencil or chalk and work them down a bit with the long board, then fill. You could also use a little guide coat to define the low spots instead.
A word about belt sanders in general. They are an aggressive tool that requires a deft touch that's developed over time and even then I don't consider them a finishing tool. The best ones are somewhat balanced front to rear so they will "float" on the platen, and don't require a lot of heel or toe pressure. The worst ones don't seem to consider balance at all and tend to dig on one end or the other. Applying pressure just makes things worse in my experience. In a perfect world, your hands are there to guide the tool, but the tool itself does the work. In many respects their utility has been replaced by random orbit sanders (or dual action, aka DA) in the woodworking world. DAs have been around forever in the auto body world. Weirdly, I have two good belt sanders and can't remember the last time I used either, but given the scenario at hand, I imagine it was the best choice.
One step at a time and we'll get you there, Rob. Bring them to Boise and I'll help, I really will.
