1965 f-100 headers
I first will try to stear you away from headers unless you are gonna be racing the truck. Running an engine over 4000 rpms needs more flow. Less than that a dual setup will handle things fine. I have headers on my FE in my 66, they are loud, hot, and kill starters. The passenger side has to be taken down to change out the starter. It makes it take hours to do a 15 min. job.
After having said all that, you will have to look for 77-79 F150 or Bronco with the M engine, for a fit. There are none sold as a fit for your truck because there were none of those engines installed in your truck.
John





