It seems to me you have two questions:1) why is the car overheating, and 2) will the addition of an A/C system add to the problem.
1) There may or may not be a problem with the thermostat ( incorrect thermostat,faulty thermostat, incorrect installation,etc.). There could be an obstruction in the coolant flow (clogged block, collapsed hose,clogged radiator,etc.). Check these things first (clogged block is the least likely of them all, so don't panic!!) Before you start spending money to check or repair RADIATOR problems, bear the following paragraph in mind.
2) By adding an A/C unit to your car you will increase the demands on the copoling system tremendously. The 67, 289 with a stock radiator was right at the cooling limits of the cooling system when it was new. Plan on updating and upgrading your cooling system if you add A/C.
I had the same combo as you, a 289, 2bbl, A/T, and I live in Florida ( hot, hot, hot all the time!!). There is very little chance that the stock cooling system will keep things cool with an A/C unit in the car. In order to cool things down, I had to go to a three row radiator (a four row would have been as good or better),a six blade fan, and a shroud. Since then all is well! Car runs cool all the time.
If I were you I would replace the thermostat with a new 195 degree thermostat, replace the radiator with a three or four row radiator, replace the upper and lower hoses, add a shroud and install a 6 or seven blade fan. This will eliminate your cooling problems now, and even after you install the A/C unit.
Hope this helps,if you need any other info, e-mail me at firstname.lastname@example.org
Very good suggestions. I would check to make sure the timing is correct at freeway speed or that you are not running to lean. Next check the hoses. They collapes at freeway speed if old. As far as radiator have a 4 row 24 inch made for your car. usually $225.00 installed. Run a 50/50 mixture and 2 bottles of Wetter Water. My needle bearly comes off the C.