How do you wire the original alternator gauge? Has no terminals only a round/strap ring with aplastic coating on it.

 

There is a single wire that simply runs through that loop, it has a bullet plug nearby, just unplug it, run it through the loop, and reconnect. I am not familiar with Econoline, but light F series had a generator and a light, Medium duty had an alternator with a gauge. It may not work if things have been modified.