I don't know what the bumper compartment door looks like on a 76. The 66 has a rear opening door with the hinge on the bumper. It opens 180 degrees so no stay open spring is needed.
The 86 has the hinge next to the body. The door opens toward the body and it has a stay open torsion spring. I went all the way down to the storage garage to take this picture, so say "thank you David."
Me, I'd find a stick in the campsite to hold the door open. The spring gets in the way of my sewer hose.