Dear Mike & Keith - Thank you for your input, it is greatly appreciated.
Regarding the postings: 1) I hope that I don't have a bad head gasket...but I'll try that type of diagnosis later this morning. 2) To avoid air entrapment, I add the coolant while the car is parked with the nose pointed slightly uphill, so that any air in the cylinder heads will be pushed forward and (hopefully) out.
Also, 1980 is the last year of the iron block V-8; it is a 450, in 1981 they went to the 380 with the aluminum block, just in case you wanted to know.
I am hoping that some of the other knowlegable people out there may have some additional ideas?