240diesel- this is very cool! Some great data on D24 engine temps!
Did you ever take any measurements towing a trailer uphill in hot weather?
I like to follow a somewhat arbitrary criteria I found in an old Diesel Technician textbook defining a "sufficient" cooling system as keeping the engine under 100C under all conditions in which it is used- including towing uphill in high temps. I think this makes sense because it would (theoretically) allow you to run with plain water at 0psi in an emergency.
Unfortunately, this is a nearly impossible criteria for the stock D24T cooling system without any upgrades towing trailers in hot mountainous areas.
I use an engine watchdog TM-2 to monitor engine temps. It doesn't log and has just one sensor, but it can be moved anyplace and has a nice audible alarm:
http://www.enginewatchdog.com/tm2.html
I also wire the audible alarm into the low oil pressure light, and a float switch in the coolant reservoir.