I suspect from what you describe it could one of several things.
Thank you for the rapid and helpful reply!
Have you checked the radiator fins on the core be grass seed or other debris that prevent proper air air flow.
None on the outside. I will check on the inside.
Mismatched components are not likely on a 318 non A/C car.
You might want to find out if the shop installed a new or a rebuilt water pump. Most of the rebuilt pumps these days are done overseas and the quality of many of them is also crap.
I don't remember the brand, but I do remember seeing the box and it looked like a new unit. The box it came in, which had been trashed before I remembered to ask for it, was light/baby blue if memory serves.
I think that a thermostat is the most likely, many of the ones sold these days at Auto Bone and Car Crap are made in China and are very poor quality. Go on line and buy yourself a Robert Shaw heavy duty t-stat. These are made here and are the best stock type t-stat on the market. (...) I would suggest changing the T-stat first and see if that fixes your problem. (...) www.flowkoolerwaterpumps.com has the t-stats
Great, Flowkooler is the company I was thinking of using for the new t-stat! Is this the one you have? Robertshaw 370-160 Series hi flow thermostat
Check the water pump for excessive bearing play. Grab the fan and try to move the unit back and forth, if you observe or feel movement of the water pump shaft, the unit is failing.
Will do.
The other thing you might want to check is for the presence of hydrocarbons in the coolant. Most auto parts stores have a tester for this purpose. Hydrocarbons in the coolant are an indication of a cracked head or bad head gasket. If combustion gasses are being vented into the cooling system this results in aeration of the coolant (gas bubbles) which reduce coolant density and cause heating.