1992 B4C, runs cool at cruise, airdam in place, correct baffles, etc. But when sitting it will over heat. Both fans are on, but fast enough? 305 stock 5 speed runs temp at first mark on gauge ?180 at cruise even on 95 degree days. Creeps up to 240 when sitting. I think the cooling system is fine due to temp ate cruise, you cannot run a/c in town or will overheat at low speeds. Could this be due to retarded timing?

 

Is it actually bubbling/gurgling when the guage reads the 240? Maybe the guage is off.

If coolant levels are fine, and your running a 50/50 mix, only other thing I can think of would be try replaceing the radiator cap - it's stupid thing I know, but can make a world of difference.