I replaced the one on my 86 with one from a 78, no problems. Just a guess but have have you checked the width of the tailgate opening with one on another El Camino? I measured mine and it is 55 3/8" between the latch plate faces. If yours had been overloaded (or wrecked at some point) and spread apart, that could be your problem. Use a come-along or similar device to pull the sides back together.
BTW, don't forget that the inner skin of the tailgate, the bedfloor, and the inner bed walls are the same thickness and pattern. if you replace a tailgate that was damaged/rusted, keep the inner skin for patches !!
all the tail gates are the same on the gen 5 elco. maybe the car is bent or the gate may be bent or both bent enough to not close.measure your opening diagonally both ways to tell if your body is off square.