So logic tells me if utilizing the thermoswitch at the radiator simply install one with a setting that takes into account the temp delta between the radiator output and motor output.
I'm stilling wondering if anyone has the thermoswitch part number so I can go buy a new one. I'd prefer to get the part first rather than disassemble it then run into a problem locating one.
I think the intellectual objection to that scheme is that delta temp of water leaving the block to water leaving the radiator is not fixed; it's influenced by the absolute water temp, the air temp, and air velocity through the radiator.
Regardless of that, if we just knew the thread of the thermoswitch it probably wouldn't be hard to find one with a different trip point. I'd go yank mine and look, but I really hate draining and refilling that cooling system.... Maybe it's NPT and you could estimate with a pair of calipers? Put a pan under it, unscrew it enough to expose some threads, quickly measure and retighten? 3/8" NPT seems to be by far the most common thread for these sensors. See
http://www.summitracing.com/search/part-type/fan-switches-thermal
The approx. OD of male 3/8" NPT is 0.675", and it's 18 threads/inch.
Failing that, I've seen kits that use a sensor that slips into the end of the rad hose and the radiator spigot. I've seen one that included a kind of double-wedge shaped rubber "spacer" for lack of a better word that fits between the hose and the spigot and provides a roundish shape so the hose doesn't leak. This link
Derale Adjustable Fan-Control Thermostat with Radiator Hose Probe Derale Radiator Fans D16769
implies you can insert the probe of Derale #16769 into the hose like this. As a temporary measure there are a lot of fan controllers that use "radiator probes" that you kind of shove through the radiator which is a mounting method that scares me. But I suspect with some cleverness you could slip any of those through the hose-spigot interface. Another advantage of this scheme is that you can put the sensor on either side of the radiator. Furthermore, some of those come with sophisticated adjustable controllers.
According to my notes our radiator spigots are 1-5/8" (41 mm) OD so maybe one of these adapters can be made to work:
http://www.summitracing.com/search/part-type/temperature-sender-fittings
BTW, regarding Jack's comment about modern PWM fan control, I just noticed Painless makes a nice looking dual-fan controller:
Painless Performance: F5 Dual Fan Controller
This assumes a 3/8" NPT sensor bung is available. I had always planned to use something like that in P2160 and will get to it eventually. They want you to put the sensor in the water jacket in the head, and to put it on the radiator only as a last resort.