I cannot answer all your questions.Undobtedly the two different era trucks will have different wiring harnesses, though.

In your case, I would consider one of the after-market flasher kits from either Signal Stat, or Yankee.They are frequently available on eBay, are similar, and are sought after, i.e., not cheap, when bought NOS. By the way, all vintage lighting products, if in good condition are rising in price.

They were made for a long time, are a universal type kit, and look period correct in our old trucks.