In the owners guide. If you don't have one you can often go the library and look in their reference section. Most libraries have repair manuals there.
There is a diagram in the owners manual (page 125 for fuses, page 129 for relays).
to download it for free.
Go to the Ford Motor Co site and you can download a complete owners hand book.
http://wiki.answers.com/Q/Where_can_you_find_the_wiring_diagram_for_a_1982_ford_e3501982 ford e350
My 1991 E350 Fuse diagram is in the owners manual.
your local ford dealership
A diagram for a 1994 Ford e350 can be found in the trucks service manual. The manual can be found through most auto parts.
diagram to replace belt on E350 1994 7.3L
brake diagram for 1998 ford escort
goto: www.installdoctor.com on internet
My 1998 E350 takes 6.4 litres, just changing last night