I had my 383 rebuilt and installed it a few months ago. Currently have approximately 300 miles on the new motor, The temperature seems to run high; it's normally a little under 200 degrees on the freeway.

200 is not too high a running temp. Changing the thermostat will not affect the running temp. it only regulates at what temp. it allows the coolant to flow out of the block into the radiator. I think 165 is too low, but that's my 2 cents.

A couple of things...more power equals more heat.  It could simply be a by-product of the stronger engine.  

What gauge are yo using to determine the operating temp? The factory gauges aren't really all that reliable to start with, and the sending units can be off, also.
I'm also gonna agree with others...200 is NOT too high of an operating temp. 220, in traffic, with the A/C on, would be "normal", imo.
Radiator/shroud seals, front air dam, and a good working fan need to be confirmed. Also, any new engine will run a bit hotter than "normal" due to the tightness of everything. It should loosen up somewhat by around 1000 miles, and eliminate that extra heat.
My question would still be, are you sure the coolant temp is correct?

The 82 comes with electric fan system (do not know if other C2 come that way) but the electric fan is not suppose to come on until the Block (i assume that is going to close to the water temp) reaches around 240.  Just for a point of reference on what GM thought about heat.  I would also assume that 240 would be approaching the hot site but not critical to engine safety or they would want a margin of error.  Just my assumptions based on what the book calls out.  By the way my C2 stays around 195 - 200 on the road.

I run a 180 degree thermo in mine & put "HyperLube" (same type product as "wetter").  Since using the HyperLube in the cooling system, temp dropped about 15 degrees.  I run consistantly around 190 (even during 4th of July parade in Philly)

OK thanks for all the feedback. I'm not worried about the 200 on freeway. In stop and go it climbs fast and gets awful hot inside the car. The gauge I have is the stock one. I've ordered an new one that will be more accurate and then I'll have better data.

If temp is rising in traffic that seems tp point more into the fan area. do you have electric fans? stock clutch fan?

I have an 82 and the electric fan is connected to the engine temp sensor on the block it is set to come on at 240 degrees.   It does not work off the water temperature sensor. 

The car did come from Texas and the electric fan was rewired to run all the time for what that is worth.  If you install it you may want to put a switch in under the dash or something to control when it come on so it does not run all of the time.   The person that had the car before me was from San Diego and had the fan disconnected but did not have it reconnected back up.  I found a wire and traced it to the engine temp switch and found that it was suppose to drive the fan.    

The best thing I did was when I changed radiators I had them add a larger core.  Big blessing  and all I did is change from a two core to a three core radiator.  The top and bottom radiator tanks are big enough to take the addition core without trouble.  If heat is an issue you may consider doing that it will add close to 30% to your cooling capability.  

The inside temp in my car goes up when stopped but that is from the engine heat coming through the firewall.  The actual engine temperature does not change much.  this may not be the answer you are looking for but maybe it helps.

If you have electric fans and have a manual switch and have A/C in your car, you need to have the fans on when the A/C is on to draw air over the condenser, important at lower speeds.