The "best" process is subjective, but most professional welders can weld almost anything almost anywhere with almost any process. That said, the ones I work with clearly prefer TIG over MIG, and MIG over stick, and stick over torches.
I suggest you find someone to teach you whose equipment you can also borrow, and follow their guidance - there are enough variables left unmentioned in your question that it's not something we can answer objectively.
That said, Ford publishes the Ford Truck Body Builder Advisory Service, which provides their body builders book and specific information on each vehicle regarding portions of the frame you shouldn't weld, how to attach certain items, and some dimensions, wiring, and other information vehicle modification businesses need as they modify their vehicles. Included are the econoline vans, so you should find a great deal of information there:
Although I now notice that they don't have information on your older vehicle. There is still some general recommendations, and you'll find the dimensions and many other aspects of the van are very similar, if not unchanged, for later models, so it may still be a good use of time to visit that site and see what is applicable to your situation.
You can call Ford and request the earlier manuals, but they apparently don't have them digitized and online.