I know O'Reilly Auto Parts (oreillyauto.com) has a compatibility link as do others. In another thread, someone said Rock Auto does, too. On the O'Reilly site find the list of alternators for your vehicle. Click on one of the actual alternators shown to see the details. Under the detail description, there is a tab that says "compatibility". Click on that. In this case, it will show "Ford 40 vehicles". Click on that to see the list. Looks like the same alternator fits "86-91 Aerostars and "90-91 F Series trucks with a V-8.

I don't know how to tell which amp rating an alternator is by looking, unless it's on a tag maybe. Sorry.