OK so...
I have an 84 sport coupe with a carbed 350. I swapped an '89 RS dash into my car and at running temp, the water gauge reads 225°~. When the engine is completely cold and you turn the key to the ignition position, it points to 100° (the first mark on the gauge). Is my needle off in the fact that at cold, it should be pointed straight down to the six o'clock position?

 

On a good day these gauges were never accurate. If the coolant isn't boiling over, it's not overheating. Make sure fan works, you have the air dam in place, and when it's hot stop in at a shop and have a mechanic shoot it with an infrared gun to tell you the REAL temp.

 

Thanks a bunch man! Turns out that I poles a 195° thermostat out of there LOL. Replaced that with a 160° and that's about what the gauge read

 

160 is a bit low. It'll be fine for the summer, but when fall returns, you'll want to swap back to a 180 or else you'll never feel any heat from the heater.