we have a 61 Apache, basically the same truck as you. There should be two small round holes in the firewall to the right of the square hole, About midway between it and the block. One round hole is above the other, they are not side by side. They are about 1" diameter. Your heater assembly, when assembled and installed under the dash, allows the core's stubs to protrude thru these holes. I at the time have my heater assembly disconnected and removed, sounds elementary but we have not taken the time to figure out how to post pics,. I will see if my wife can e-mail some pics to you today or if we can tonight.I am sure Tulsa's winter weather is not much different than here in little rock, heat and defrost would be nice in the winter on nice days.
