Can anyone tell me what the original factory size tire and diameter was for a 1972 El Camino (not SS)?

First, welcome to ECC Roger :-) The info I have is from the chevy resto packet. It shows a F78x14 with a static loaded radius of 12.4 and loaded rev/min @ 45 mph of 785. I also have a 72 owners manual that shows either a E78x14 or F78x14. Hope that helps.