Thanks, all. Another short update.
The bottom of the fuse where the fuel tank is has some detail. Here, I've used the PART PE straps from the SPAD VII set. Unfortunately, the VII only has 2 straps where the XIII has three. The tank has 4 long bolts holding in place as well. There is a drain in the sump area, too.

Some masking, getting ready for paint.

I have installed a spreader to widen the front of the fuselage sides to match the width of the cowling. It's a bit overkill, but I used what was handy.

The bottom pan is installed. One can see wood framework around the edges of the tank, so this area was painted brown.

The cowling set in place to check the fit.

I haven't come up with anything special for the louvers, so they may just get some black paint on the open ends.
Thanks for following along.