I've done a heater core in a 720, and don't recall s screw behind the heater assy. Did you pull the dash? I'll check a service manual tommorow to be sure, but I don't rcall a screw there.
Background: I have a 1985 720 King Kab 4x4 with A/C and the heater core is bad. I've followed both Chiltons and Haynes for removal of the heater assembly and have run up against one point of attachment BEHIND the heater assembly that appears impossible to get to. The exploded diagram shows a self tapping screw that goes through a tab at the back of the heater assembly.
How long did it take you to get it apart and where did you get a new one