The bulkhead plug, on the engine compartment side, is actually several plugs that fit together. Most of the bulkhead connector pieces are discontinued. The discontinued 1987 forward lamp harness connector is Delphi Part # 8917233. It takes Packard 56 female terminals. You can buy the terminals but the connector is only available if you want to buy an entire production run of several thousand or find one used.
You can find a PDF of the 1987 GM Light Truck full line Wiring Manual in one of the folders linked in my signature. The 1987 squarebody is an RV series. The RV series forward lamp harness is on page 167 & 168.