GM-GMC: P30 Cooling System, grumman stepvan, coolant temp

Question
How do I completely refill the cooling system on a 1974 Chevy P30, 350 engine, Grumman Stepvan when there are two heater cores (one under hood and one inside the cab)mounted higher that the radiator cap?

Answer
Just for the record, I wasn't even born yet in '74. LOL
There are a couple of things you can try.  One is to raise the front of the van to help the air to escape, the other is to fill the system as much as you can, fill the overflow, seal the system, turn the heat on full and start it up, while its running crack the line on the downstream side of the heater core untill coolant comes out steadily, close it off and run it to operating temperature.  Watch the coolant temp on your gauge.  If it gets too hot, shut it down and let it cool off.
Hope this helps