I have stripped my 1969 Mustang down to bare metal and treated it with "Capt. Lee's Matal Prep & Conditioner" which leaves a Zinc Phosphate coating which keeps it from rusting. It's now ready for Primer. My question is, what kind of Primer should I use? I had planned on using PPG-K36 Acrylic Urethane Primer Surfacer, but I friend suggested using PPG-DPLF Epoxy Primer first and putting the K36 over that. Is that the best way to go, or will the K36 alone do the job? Thanks in advance.

Yes, use the epoxy primer first. Epoxy primer is a self etching primer that adheres itself to the metal surface. Then you use your primer surfacer.

Primer surfacer doesn't etch itself on, and you'll have problems with your paint staying on the metal. It can lift, peel, crack, and so forth.