The harness is a big part of the EFI system.
If you study this web page:
You'll notice that the 300 / 302 / 351 / 460 from 1987-1995 Trucks, Vans, and Broncos all use a harenss very simpular to each other. You can use your original engine harness, so be very carefull with it as you remove the old engine.
If you had a 302 you DO NOT need to make any modifications to the harness.
If you had a 300 you will need to adjust the fuel injector connectors for the V8. It's not really hard work, just unwrap the harness and strech it around, then...
Piggyback one injector to the original #1 injector.
Piggyback a second injector to the original #2 injector.
The good part about this EFI harenss is that the connectors will only fit onto the corect part. So now install the engine and lower intake. Hook up the injectors, then strech the Air Charch Temp sensor and Engine Coolant Temp sensor plugs to reach the new locations.
Now get out the upper intake with throttle body installed. Notice all the connectors on the throttle body and look at the connectors left on the harness. Get a rough idea where you need to strech the remaining connectors to.
If you had a 460, Why are you installing a 351