Welcome to 70 LWB club!!! I started basically where you are. Taught my self how to weld, work that body, and paint while restoring my 70 (still learning really). There is so much information on the web you can learn anything you need. Just take your time and you will do fine.
As for going to bare metal. If the DA is taking the paint off easy, its flaking off, or not feathering on the edges, then I would take down to bare metal, as that is a sign of bad adhesion. My truck had a spray can primer on top of the original paint and I didnt take it to bare metal on all panels(most but not all). Inside the cab I just sanded to get a tooth to stick to. I knocked the primer off real quick and could tell the difference between what was bad adhesion, that I didnt want to paint on top of, and what was acceptable to me.
Get some sheet metal from HD or Lowes and practice your welds. Spot welds and butt welds are the most common. I like to test my practice pieces by taking them back apart. Your screw ups can be great learning experiences, not that your going to have any of those, right.
What color are you going with?
I love going to the home improvement store with an 8' bed. Good luck.