Since that piece fit well, it was time to put the outer quarter back on to see if I really am getting this thing lined up. Sure enough, everything seems to be good. Just need to get a measurement from the tailgate before I make that bracket placement permanent.
I also started messing with making a filler using Mike Moore's piece as a buck. You can see it sitting in the bed in this shot, with the outer clamped in place too. Once I get that filler piece put together, I'll start tacking all these big pieces in place. With any luck, that might just be tomorrow.
Keep your fingers crossed. Up to about 87 hours now.