So I buttoned up the underpan, and tried the car again the other day with ambient temps around 90F. There were several complications including a leak in my Filter King fuel regulator on startup and the detachment of the coil wire while underway.
Bottom line: Oil temps reached 220 in about 50 minutes driving the same roads as before.
I went ahead and rechecked the valve clearances (they were good) and pulled the cover off the oil cooler on the off chance there was an obstruction (nope, all clean). I carefully arranged the spark plug booty thingies over the holes in the fan shroud and drove the car again today for an hour in ambient 80F.
Temp rose very slowly this time and ended up around 210F.
Oil pressure is about 45 lbs at 3000 RPM at that temp.
I've not yet allowed the oil temp to plateau or stabilize at whatever its max is going to be, but after an hour of stop and go driving I think my readings should be getting close to what they'd be. I think this means I'll need to take them down 20-25F to make the car right.
Plan now is a Mocal sandwich adaptor with a 180F thermostat, AN 10 connectors on braided steel hose and a Mesa 48 plate cooler in the right rear corner of the car, mounted between the trans crossmember and the rear crossmember. At 6x11 inches and 1.5 inches thick, that's about the max of what should fit there, and I may have to re-route the small shifter cable. Looking at a 7-inch low profile fan and a 190F thermostat to put on top, under/next to the spare tire.
Looking at the stock oil cooler in the fan shroud I'm thinking the Mesa 48, arranged in that way, ought to do the trick.
As always, any advice/wisdom appreciated.