Here is a strange one for you. I recently put a new exhaust on my tow truck and now it runs hotter. The truck is an 85 Chevy K20, the truck originally was a 350 with manual transmission, it has the higher Gross weight of 8600 lbs. It came with a full stainless exhaust from the factory. I was told by the exhaust guy that the truck was designed to run hotter with the heavy duty engine that it had. The mileage is about 180k, the current engine is a GM Crate 350, it was in the truck when I bought 20k miles and three years ago. The new exhaust is a dual exhaust, I also had the exhaust guy put on different exhaust manifolds. The new manifolds are from a late 70s chevy truck. They are cast iron manifolds, completely stock. The old ones were a stainless steel tube style manifold, again the stock options. The old ones had exhaust leaks where one of the previous owners had tried to remove the some kind of exhaust gas recirculation system. So the new system is a dual exhaust, with flowmaster series 50 for mufflers. It is nice and quite, but now the temp runs about 210 when tooling down the highway, where previously it was 180. If I let the engine idle, like at a stoplight, the temp drops about 10-15 degrees and then goes back up once under way. The only change has been the exhaust since this occured. I can't figure why the temp would go up with a new exhaust.
Any ideas? It doesn't seem like a big deal to me, just a puzzle. In the next couple of months a I am going to put a mild 383 in the truck, that is when I got the new exhaust.

Its probably just a coincidence that its getting hotter. I would bet you have a thermostat sticking a little or a radiator that is getting clogged. Also may be low on water. Ive never seen a vehicle heat up because of exhaust work. I have seen clogged cats cause power problems, Im assuming the cats are long gone or new.

A side benefit of the 8600 GVW rating is that it was exempt from catalitic converters.

A more restricted exhaust system will cause a motor to run hotter. Maybe the exhaust manifolds restrict the flow more than the old ones.

Pull a plug and see what color it is, if it`s white it`s lean, less back pressure from a previous exhaust system can cause the mixture to be lean resulting in hotter temps. If the truck has a vacuum advance I`d check and make sure it`s working.