68 camaro overheating problem

I just added the Vintage A/C system to my 68 camaro and really love it , but now I have a overheating problem while sitting in traffic. Was wondering if anyone else has run into this. I have a 350 engine with a 4 speed transmission. The first radiator was a 3 core for a automatic trans so I switched it out to a 4 core for a manual and that helped a little but the car will still overheat while sitting. It runs very cool while moving but will slowly overheat while sitting still. Any ideas I would appreciate. Thanks , Art

hey thanks for answering , yes I have a fan shroud and a 6 blade 17 inch fan without a clutch. I will check the pulley size when I get home, what size should the pulley be? I wouldnt think the thermostat is new probably several years old cause I have never changed it since I have owned it, what temperature should I put in it and I will replace it.