I just solved this problem on my "new" old columbia I bought, I was trying to mount 26 X 2.125's and the fender wouldn't fit. First, add a chain link to your chain to get the wheel as far back in the droupout as you can, mount your fender to the hole by the crank tube only, and sit the back wheel in place. Find yourself a couple of screwdrivers or whatever to use as spacers, and set up the fender so that it has clearance from the tire, and measure the gap between the fender brace and its mounting hole. Then take a couple of scrap pieces of aluminum, and make yourself mini truss rods to bolt to the frame, and the ends of the fender brace.
I'll try to get a picture of what im talking about up later tonight.
Rick