I've done Fords too. They have small rods from handle in middle to gate latches. Many times those little plastic clips for holding rods in handle holes are broken or missing. You can remove the handle assembly, and pull on those rods by hand or with tools to open.
Not sure if chevy had that or similar? Would think it would have to be. Maybe a cable instead of rod? Remove the center handle first, look around in gate for loose parts if nothing was on handle to disconnect.
As said, about any thin tool you can push between gate and box sides and push in the latches should work. Not too handy for all the time, but to start should be ok. You can still get those handles and parts easy for those years. Lots of venders around, check the Links here on this site. Order a manual also, should have your info in there.
