truck info .. originally 1991 4.0 auto

So im to the point now were I have to make the truck and the motor get along .. I have a complete engine and tranny harness and the comp ..

I have a haynes repair manual for the mustang that the harness came out of and one for the ranger as well ..

Now stop me and tell me if im wrong but can i just cut all the 4.0 wiring off the old harness and leave all the lights and stuff in that bundle and then run the 5.o stuff on its own since i have the firewall gourmet as well .. ??

P.s I did use the search but i got the message that the words I was searching for were to common