Incrementally drilled the four mounting holes for the stab. This required several iterations of clamping, drilling, stab removal, re-measuring, and adjusting. In the end, I think it turned out well. Left the stab attached with 3/16" cleco's for easy removal.
While the stab was off, I did some additional edge clean-up on the mounting bracket to resist cracking, given the Zenith mandatory inspection information.
Riveted the starboard gusset in place, but left the port side cleco'd. I'm not riveting the port side rear fuse panel holes so It can be removed later for ease in routing cables and such.