I would like to know what is the recommended thermostat to run in my 1969 427 390 hp.  The engine was recently rebuilt. I am not sure what type was installed.  The car is driven in Virginia and currrently the temp is around 200/210 after normal driving.  I have seen either 160/180 degree options.  I am not sure if this will help?  Any suggestions

My 1974 Corvette with the 454 in it always ran 200-210 when I purchased it during the hot Kansas summer months, so I switched it out to a 160 thermostat with a bottle of "Super Cool" (get at any parts store) added and only a 25% mix of antifreeze (water is a more efficient coolant), and now it runs around 180-190 in the summer and 160-170 in these cooler winter months. Hope this helps!

Thanks..  I will look at getting a 160 . May have to try a couple of different ones to see which works best for my engine. 

Changing to a lower rated thermostat will NOT cause/make the engine to run any cooler, in itself. It only changes WHEN the coolant starts to circulate. A thermostat that opens sooner will get the coolant circulating sooner, which can sometimes help with temp issues. The entire cooling system has to work together to keep engine temps in check. Big block run hot....period. Put one in a small, cramped engine bay as the Corvette has, and the problem just gets worse.
Make sure all the seals are present on the radiator support, around the fan shroud, and across the top of the rad to seal to the hood. Make sure the fan clutch is in working order. Of course, the water pump must also be in good condition. Be sure the lower air dam has the rubber "extension" on it on the '68-'69 models....very important piece that sometimes goes un-noticed.
All that being said, 200-210 is not all that unusual for a big block....but 180-190 would sure be better.

Thanks. I will look to make sure these were put back on when the engine and radiator were out of the car.   

What radiator are you using?

I'd put a 180* thermostat in it.

I have the original radiator in the car. I had it flushed out, cleaned , and tested with all new hoses and belts installed when the engine was overhauled. 

Those guys brought up good points in that the radiator must be sealed well around the support and the fan clutch and fan shroud must be in good condition.  All of that was fine on my car and I agree that my 160 only difference is that it opens sooner and gives the radiator/engine sort of a head start on cooling.  THE BIGGEST CHANGE was that I always used way too much antifreeze and not enough water so I changed over to only 25% antifreeze and 75% water with an added bottle of "Super Cool Water Wetter" which allows better heat transfer between the water and the metal radiator.....all I know is it worked for me.  My car is always garaged and never left in a freezing situation so the lower concentration of antifreeze doesn't bother me.  The Super Cool also contains a water pump lubricant.  I added an additional front air dam that scoops up air and put in a center grill instead of having a front license plate so there is more air flow coming through.  I still believe switching to higher concentration of water (I use distilled water) and using Super Cool made the biggest difference because standing still at idle, she still stays (relatively) cool.  Hopefully some of this will help your situation also.

Thanks, I checked today and the seals are in place around the radiator. The fan clutch is new. Interesting idea about the mixture ratio.

ALWAYS use distilled water.  Especially if you have an aluminum radiator.