i've had two 79 LRE trucks over the years. both very original/ no air pumps.. they did have some kind of air pipe to the rear of the passenger exhaust manifold.. i forget where it hooked up to though... as far as the cats go.. they are over 40 years old. probably shot any way..
My Magnum had a passenger side pipe that was held on by 2 bolts. The tube went to a check valve so there was no backward air flow. I think it was called a ”Diverter valve.” The hard steel line continued to a rubber line attaching to the air cleaner.
So as long as the carb was running right and the lean burn was working correctly, the exhaust gas should have been mostly inert gas. While it wasn’t good for power or performance, it otherwise generally didn’t hurt anything.