Instructions say shim as required, but I didn't want to just cobble this together, so I took some time to try and figure out why the gap along at the aft flange of WD-409 wasn't constant. Ultimately, it came down to 2 issues. Basically, the F-712 was slightly bowed to start with and when I clecoed it to the skins it just made the fit worse. Got the face flat by diligent use of a lead hammer, and then made sure that the flanges were at exactly the right angle so that the side skins didn't tweak it when clecoed back on.

Took a few trial fits, but finally got the gap between the F-712 and the WD-409 relatively consistent across the top edge. Unfortunately, it's not constant vertically. The gap is bigger top than bottom.

Ended up with a somewhat tapered shim to make everything fit right. Specifically, it's two .025 shims that cover the entire mating surface of the WD-409 to F-711 interface, and one .020 shim in between them that only catches the top 1/2 of the same area. This results in a tapered shim that's .070 at the upper edge, and .050 at the bottom. Success! There is a tiny gap at the top edge of the Weldment to F-712, but it's only about 1/2" deep, and I think that it will all pull together nicely during final assembly.