I have to chime in here because I recently installed a 5.0 out of a 1992 into an 1985 former V6.
It has been awhile since I did the wiring but I didn't need to splice anything if I recall. I found the headlight harness for a 1986 5.0 and plugged it in under the sterering column. I then found a 1986 ECU harness and plugged that into the headlight harness.
Now when we installed the motor I was thinking abouth running wires to the fuel pump and inertia switch but my father who installed the tank said that he plugged everything right in. I didn't understand since the V6 had an external fuel pump that I removed and the conector for it was under the car near the rear seat.
Well, when we were ready to fire up the motor I had my doubts about everything working because of this fuel pump issue. Sure enough when we plugged in the computer and turned the key the fuel pump kicked in. The car has been running for several months now and reading this still makes me wonder why we didn't need to do any splicing. I have the green connector near the ECU just sitting there not connected to anything. I am at a loss how everything worked.
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