Here's what i'd do. Cut out all of the loose flaky rust. Leave enough rusty metal there to support your bodywork. Coat all of the rusty area with your favorite rust paint (Rust Encapsulator, POR 15, etc.). Bend the edges of the rusted area inwards about one inch around. Then apply some fiberglass "sandwiches" to the area. (This is a layer of fiberglass mat with resin on each side.) The can of resin has the instructions. And yes the fiberglass will stick to the rust encapsulator-I can vouch for that. I'd never apply fiberglass to bare metal. Build the area up to contour. Sand it out roughly to match the contours. Then finish it with Bondo and prime and paint. Look in my gallery. See my dad's orange '74? I did it about 6 years ago and it is just starting to crack and pop though. From 20 feet away you'd never know. However on my '75 I am currently putting new metal in. I learned to weld and metal is a far superior repair. But if you are not going to keep it the call is up to you. Also, i've been trying the new "glue/rivet in" panel repair. Works well. I put my '75's cab corners on that way. Worked like a charm. But I can't confirm the durability yet. Always use fiberglass resin and matting to fill holes. The resin won't absorb water like body filler.