For '86, the ECM is located behind the passenger side kick panel inside
the cab. On the passenger side fender in the engine compartment is a panel that contains the starter solenoid and other relays . . . some of which you might want to keep (such as in-tank fuel pump relay).
I would suggest obtaining a Hanes manual or something with an electrical schematic covering your year to help you determine what you need to keep and what you can toss.
BTW, I have an '86 Ranger w/351W. It is an extremely tight fit. (Check out some of my pictures in my user album).