My 1990 E350 has 235 85 R 16 size tires.
6890
E4od
31by 10.5
Your tire dealer can help you with that.
P245/65r17 105s
You can lower it with the help of a mechanic. You can take it out and then place it onto the empty part.
225-60.16
P185/75R14
1991 e 350 diesel 7.3 has 190 hp
Just use the suggested R15
http://wiki.answers.com/Q/Where_can_you_find_the_wiring_diagram_for_a_1982_ford_e3501982 ford e350
235-85 16