I use Motorad Thermostats, the stock unit for the 300Tdi is Pt# 319-192.
You also need to buy the rubber gasket that fits the 319 body, Pt# MG29.
The last 3 digits indicate degrees fully opened. You can find these T-stats on eBay.
Motorad also has a stock unit 319-180, which Javelina Dave utilized in Phoenix to keep his running cooler in that hot desert environment. I use a +- 2 degrees IR gun to test these units and their stamped temperature rating on each wax pellet.
Building them hotter is just a matter of getting a stock unit (329-192) and getting one of their T-stats with a hotter rating as it's the wax pellet (the copper "thing" with the steel stud sticking out of it), disassembling them (can be done without cutting them) and reassembling the 319 "body" with a hotter wax pellet. So I'm currently running a 319-203 (203 degrees) T-stat and have also built 319-195, 319-198, 319-203, and 319-205. The wax pellets have the temperature rating stamped on the bottom of the pellets. When reassembling the components pay attention to the OAL height of the reassembled unit as the bottom of the T-stat closes a secondary port in the housing too. Be sure to put the "jiggle pin" or vent hole at the top.
Changing the T-stat is quick & easy on a cold engine. Remove coolant tank lid,take the small cap off the radiator, shove a 3/8" diameter by 5' length piece of clear vinyl hose in the rad until it stops (about 1' in), siphon away and drain into a clean 5 gal. bucket. when it stops draining you've got the level down low enough so when you open the T-stat housing (2- 8mm bolts) maybe you'll lose about a tablespoon of coolant. Install new or different T-stat with the rubber gasket, pour coolant back in radiator (take coolant reservoir cap off for ventilation) or in coolant tank. Put the lid back on the rad & coolant tank and you're done.
Something to note is that some Ford diesel pickups use a 215 degree thermostat, so a 205 is not pushing any sort of limit though when I run my 205 my VDO gage says 209 degrees, and over 210, I am not comfortable to go there, so I stick with the 203.
Hotter diesel combustion chamber temps do cause higher NOX emissions, this is why the EPA uses cooling systems on modern EGR systems to lower the EGR gas returns temperatures which puts more carbon particles back into the intake which most believe shorten the lift of a diesel engine, can't be good to put that crap back into the intake air.
Gas engine combustion temperatures run much hotter than diesel.
Diesels make more heat by burning more fuel and retaining it in the coolant, block, etc.
Look how small your radiator is compared to a gas engine.
Off road using lower RPM on a diesel and engine temp's will run low, in a gas engine you're more likely overheat at slow speeds.
PM me if you need some more direction on building hotter T-stats.