Hope this works--looks exactly like mine. Very hard to find find under the hood! It is more on the passenger side below behind a bunch of stuff. Be a chore to get it out.
The link should jump to the 460 photo after maybe a quick peek at the first page of images.
https://www.google.ca/search?q=1991+E350+alternator&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwiEqt3K04zYAhUBI2MKHQwbDEoQsAQIYQ&biw=1093&bih=470#imgrc=_7wCkR9NIJ7D_M:&spf=1513362844740My engine battery is in the front passenger side corner under the hood and the isolator and CB to the house is right beside the battery on the right side.
I just got a new starter (cost a bit!) and a new air hose to the catalytic converter. The big air filter on top needed a clean.