Man, I had fun reading your post.
Those are some impressive ETs. You ran 11.6 @ 116 on 7 cylinders.
Driving skill can truly make the difference--and you seem to have it, ...also the intelligence of trial and error. Knowing myself, I wouldn't have even thought to bring extra plugs (but now I do).
Seems like those ET streets are treating you good though. But how 'bout next time not going lower than 12psi.
...when you said "I changed the plugs as it started to rain briefly. I am ready to have another go, and just as the track is almost dry the rain comes down in sheets," I was not surprised at all though, that CRAP happens to everybody-- especially me
...but I can't imagine 30 mph for 100 miles... ON ET STREETS...
IN THE RAIN! That's gotta be the worst combination.
-----------------------------------------
351W-powered 1979 Ford Mustang notchback
Stock 5.8L under custom made 4" cowl, C4 w/ shift kit
Holley 750 cfm, Edelbrock Performer RPM intake
1 5/8" MAC shorty headers, Al driveshaft
2.5" Off road H-pipe, 2-chamber Flowmasters
Front: 225/60/15, Rear: 255/60/15 Eagle GT II
Weld wheels (15x6;15x8), 8.8" Rear w/ 3.55s
14 x 4” K&N air filter.
Only a couple of weeks b4 Project FOX begins!!! 
"Red, thou art my companion. Hasten now your quickened metamorphosis to Green that I may conquer all who dare abide there beside me. May they be left thither behind burnt black." ---Fox Body