I have been working on my truck for the longest time now trying to figure out how to adapt the harness off my 1992 f150 auto to a 1985 model 351w 5speed anybody know an easy way to do it? Their both efi but there is different connectors on the motor and on the fire walk

 

Do not use any wiring from the donor engine. Keep the wiring from the original truck.

 

how would that work with the different connectiors just splice them onto the 302s harness? Also how could i bypass the manual spedo cable to still use electronic?

 

No way that 351 is an '85 if it has EFI, did you mean '95? In any event the advice given is correct.. DO NOT use any wiring that comes with the donor motor, strip it ALL off and install the engine harness that was originally in your truck.

 

I am still a bit confused here. The truck is what model year? The donor engine is what model year?

 

The boy i got it off had it in a 85 f250 but it is a efi model said it was probably a newer motor which it should but couldn't give me the year that it was the only problem im trying to wrap my head around is the speedo electronic 4x4 actuator to a manual in the floor and some of the things like the distrubutor has a connector unlike my 302 harnes that is wired straight through

 

So bottom line is you can't use any wiring that came with the 5.8 motor. Remove the upper intake and take it all off, then do the same to your old 5.0 and put that harness on the 5.8.
Now the different distributor connector tells me your 5.8 motor is from a newer truck so your seller isn't lying there, but that also means you have a little work to do make it work. You have 2 options, go find another 5.8 distributor for an early ('91 or older) 5.8 motor, or dismantle both of your distributors and transfer the PIP module from the 5.0 dizzy to the 5.8 copy. You also need to carve a little slot to connect the TFI module to the PIP and drill and tap a couple holes to mount the TFI module if they don't already exist. Then ideally you want to find a PCM for a 5.8+manual trans and plug that into your truck.

Your truck has an electronic speedo driven by a speed signal originating at the rear axle so you don't need to change anything there.
Are you changing the t-case? Why don't you use the original t-case that was in your truck?

 

I was going to use the transfer case but its bolt pattern looks different