Classic/Antique Car Repair: 1949 Buick 248 carburetion, lean mixture, rich mixture

Question
How do you do Brad.
I have recently acquired a 1949 Buick Super Sedanet, Model 56-S.
248 ci engine with Carter WCD carburetor.
The previous owner has assured me that the car ran well in his care and is freshly rebuilt with recon water pump, fuel pump, rebore and crank grind etc.
The car came from Colorado and was being used in an area at about 7000 feet above sea level.
The car is now being used at about 200 feet asl.
She will start and fast idle but will not rev above 30mph in top gear without stuttering and vibrating badly.
overheating is occurring if left to idle for more than 10 minutes.
the temp gauge shows normal if the car is moving along.
If the previous owner is believed, which I hope is correct, then I am wondering if the carb may be fitted with the high altitude metering rod that was available for this model.
The carb looks like it could use an overhaul anyway.
Any advice will be much appreciated as I could be spending a lot of time working this problem out!!
thanks.

Answer
The running problem appears to be more than just a lean running condition. I would start diagnosis by removing the spark plugs and reading the color. They should be a light tan on the firing tip. darker indicates rich mixture and lighter indicates a lean mixture. I would also verify the engine with a compression test before doing any thing else. The overheating at idle concerns me. Does the car have the fan belt driven fan or has some one added an electric fan? Let me know.
Brad