[swamp rat]

I also have a 72 and my return to the radiator actually on the bottom of the core yet Everything i have read says to have the core bottom the inlet and the top the return so it fills from the bottom. the core inlet and outlet are different sizes so i was guessing that maybe someone at the factory may have installed some of the cores upside down?

In your case the line leaving the manafold is a positive pressure and should be feeding the core and should go to the bottom, the top should return back to the right side of the water pump just above the lower rad hose, or if you do have a return port on the radiator it can go there..

Some trucks didn't use the radiator for a return and had no port so they returned back to the water pump.

EDIT: +1 ^, Take both hoses off at the engine and shove your garden hose into the end of the large heater hose and turn the faucet on slowley and see if it pushes crap out of the core. Also you may want to concider pulling the cowel and wipers and cleaning out the heater intake from 40 years of crud buildup.