Engine running too cool...totally baffled!
You should be able to touch the valve cover at operating temperature but if you can keep your hand on it when it is at operating temperature a vapor lock could be the problem. It stands to reason that if you run a 190 thermostat for example, the motor will run at 190+/-. If you have not done so yet, try spending a bit more time burping the system to get rid of the air bubbles.
Hope it works for you.
Are you running a carb or EFI?
If it is a carb, I would go to a 190 thermostat. If the engine runs much higher you'll get performance troubles. Especially when you go to restart when it is still warm.
If it is an EFI, it's OK to run the higher temps.
Where is the temp sensor located? Is it on the side of the block or at the top. The side of the block will usually give you a higher reading than on the top.
By the way, a 383 is nice in that truck.