Update: It's good.
I'll mount it in there tomorrow with a dab of silicone goop on the beam and lock washers on the nuts, and wire it tight.
A reminder: "perfect" would be if the thing didn't flex in the first place, like if I welded in proper gussets when I altered the bulkhead for the clutch pedal.
I think this fix qualifies more as an "elegant bodge."
I appreciate everyone's patience and encouragement over these past three years. It's the 11th hour of the build and my welds are starting to look "not totally incompetent," which gives me a feeling of great accomplishment and manliness.