Also make sure the battery terminals & connections are clean and you have the proper old size battery cables and not the 12v ones or even those sold today at parts stores for 6v. It is also possible the starter is getting heat soaked and losing efficiency & it or solenoid needs repair. I just replaced the starter/solenoid in my modern Ford PU for the exact same problem-worked cold, clunk but no go when hot.
As to the wiring harness, if the original type repro is out of the question now, individual wires of the proper type & color can be bought by the foot from places such as RhodeIsland Wiring.
http://www.riwire.com/ Difficulty with that approach would be the wire would be external or else the fabric covering on the loom would be cut & lost if removing the old ones. They also have the proper size battery cables if yours are suspect.
There are some modern universal aftermarket looms that others here have used with success. Most of those are designed around 12v though so a few changes or relay additions might be necessary with heavy draw items such as headlights. Couple of those are:
http://www.kayjayco.com/techKits.htmhttp://www.painlessperformance.com/This Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=37448