i know the computer must be changed for this but must the wiring harness be changed out as well

 

In what?

 

2002 f 150changing from a v6 to a v8 must i change the harness in the cab as well as the computer

 

What transmission?

 

I think there's a LOT more involved than just a little engine harness. This will probably be tougher than a 4.6 to a 5.4 swap for a lot of reasons, but who knows. I wonder if the trans will even mate up . . . well above my knowledge realm for that year. BTW, I think 4.6 versus 5.4 may matter too.

 

The engine computer, yes. Dash harness, no. Unless you want to swap to the fancier lariat setup with the EATC and whatnot, then that's gonna' have to change.

You'll need to change the transmission no matter what, but at least going from 4.2 to the 4.6, you can keep the rear axle and radiator. Every little bit helps.