How many miles on the engine? The cap will prevent it from getting very hot. The old cars didn't have a pressurized cooling system. I'd check the engine timing for slop and maybe do a compression check on the cylinders. If our timing chain has a lot of slop, it will run cool and your fuel mileage will suffer. If it has a lot of slop, when you replace it, you will get much better power, heat and fuel economy. Only if it's bad though. Turn your engine by hand until the rotor is at number one position. With the distributor cap off, use a soap stone or a sharpie to mark the position of the rotor. Do NOT use a pencil as the graphite can cause shorts and misfires. Now rotate the engine backward by hand. If it moves more than 5 or more degrees before the rotor moves, your timing is off. A new timing set will take care of that.

 

Did you use a gasket when you installed the t stat? Some people have had the the coolant bypass the t stat and it never gets to operating temp.

 

Engine has 186K miles on her. The timing chain did have sloop, replaced about 10k ago.
Thanks to everyone who provided feedback.
The truck is running like a scolded ape. Here's what I found.
​​​​​​Like a DA, the thermostat I installed, number 2, was a 185. Replaced with a 195.
Replaced the rad cap, now the system is building pressure. But still running cool.
Replaced the fan clutch. It was leaking and was not dis-engaging at cooler temperatures. Running the fan even when cool. She now runs better than ever. I'm in the middle of restoration. Finally found a set of original gauges that test good. I built a gauge tester per the shop manual. I finally, after 3 units, found a temp sending unit that has the proper resistance (300 ohms cold). Replaced the old voltage stabilizer with an electronic one. Temp, oil and amp read per the manual but the fuel gauge is off. Can't find a fuel sending unit for my 88. The 88 is a bastard year, some parts were only made for that year. If someone has figured out how to to use another sending unit, I'd be greatful. I've learned a lot about the instrument cluster if someone needs help.

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Not sure if this will work for a 88. Google search this, I posted a link but it won't work.

"how-to-retrofit-newer-fuel-pump-assembly-to-1989-1990-dodge-ram-gassers"