I could maybe get one out of the E series since that is the same. I would expect the harness is rat chewed quite well by now... the plug should be fine.
I told him to check the yards, but since '77 had its own ignition switch, had said he can't find any. He just needs the plug that connects to the ign switch with the wires cut off a couple inches so he can connect them up. Heres already got the ign switch though..Some idiot cut the wires on his.
I wasn't aware that they had so many changes to something so simple back then... He might have to go to changing the ignition switch and get the connector with it. It likely is just the arrangement of the plug, not the wires that is different.
The switches actually differed with the different models in 77. My truck is a F-150 custom and when I put a cab on from an F-250 Ranger xlt of the same year the switches were different, I could not plug the old switch into the wiring on the new cab. Look online someone should have it, lmc has the plug for 73-76. or if you figured out how to wire it you could use the plug and switch from a different year.
Here is the listings for LMC Igniton swithces
73-76 part # 47-0724
77 is listed all by itself #47-0727 this is what i used when i swapped from my 79 Harness to a 77 Harness.
78-79 #47-0728
73-76 Wire Harness connector #47-0719 this is the only connector they list