"I recommend you get a digital
"I recommend you get a digital infrared laser point pyrometer. You'll find many uses for it. I use mine to check the circuit breakers in the middle of the winter to see which ones are heating up when the heat is on. I also use it to check my various automobiles (I collect water cooled Porsches) and to check the AC in the house and the cars. On a boat this is invaluable. You can pinpoint a hot spot, and you can verify the temp reading on the motor too.
I don't know what the 283 temp reading is to be, my manual is at work and I'm at the house at the moment, but even the big 427 is supposed to run 130 to 150 on a thermostatically controlled motor. The 283 isn't a thermostat motor, so when running at less than wide open throttle, it may well be running very cold.
It would be good to do a check with the pyrometer. Just look at what your temp gauge is reading on the boat, and point the laser point pyrometer directly at the sending unit. It will tell you exactly what the temp of the sending unit is (it should be slightly cooler than the internal water temp).
Regards, P"