I drive a 78 f100 and i want to make it a 4x4 depending how hard it’s gonna be. Ive got an 86 f150 parts with a 4 speed granny, transfer case, etc. with a 8.8 rear end same as mine. How hard would it be and what modifications would I need to make to make the front end bolt to mine. I already know I have to cut a new hump in the floor

 

How are your fab skills? The trans and transfer case from the 86 can be used but it would be a mess to use the front twin beam axle off of that unless you wanted to cut the frame at the firewall of both trucks and weld the front of the 86 to the rear of the 78.

Another option would be to put the body of the 78 onto the frame of the 86.

The cleanest way to make your 78 a 4x4 would be to sell the 86 and buy a 77-79 f100/150 4x4 frame and running gear.

To answer your main question... it’s hard! If I were to convert a 78 I wouldn’t use anything, axle wise, from 80 and newer.

 

Some words on if from fellow a FTE member.

"So you want a make a 2wd into a 4wd....There is nothing nuts and bolts about a 4wd conversion, there is not a thing that is bolt on or swappable. The engine cross members are entirely different, and this means that if an un molested truck is what you are after, it will be impossible to create one with a 2wd platform. The 2wd will have to be lifted quite a way to clear the front diff. 2wd trucks are coil sprung and most F250 4wd's are leaf sprung. None of the suspension items are even close to the same, so you will be fabricating or modifying some original 4wd stuff to work.

Steering systems are entirely different as well, so you will be upgrading that too. My last conversion with leafs ran me several thousand dollars, and that was with the work that I performed for free. The difference in sale values between a 4wd and 2wd are minimal, and the 4wd conversion is hardly justifiable. Well the frames are similar, with a few exceptions. The frame horns are slightly different, and the frame widths aft of the cab are different from 73 to 77 trucks. The frame rise is also slightly different, but this is not a show stopper and does not make much difference one the fabrication begins.

Now the important question is whether or not you will be using a coils springs or a leaf spring. It is possible to install a 78/9 radius arm and suspension. Not a big deal really, and with a standard cab this is reasonable. Not quite as easy with a super cab and or crew cab.
The 2wd frames are not cut for the shackle pivot, and they do not have the provisions for a spring hanger, but again, this is not a deal breaker. Installing a pivot is just a tedious process, accuracy is key.

Ok, so next is the transfer case. You have two choices, install a married case, and this requires the proper transmission. Might find some difficulty making or modifying an output shaft to accommodate the early C6 and a married transfer case. So then the next option is to find a divorced transfer case, and install this. You will have to make up a cross member, but then you will have to get creative with the shifter linkage. See the 4wd trans will have an adapter or tailshaft with a pivot for a typical 205 shifter. Fabrication is the only limiting factor here.

Next and final factor will be the speedo. See your speedo works from the trans, so that being the case, you will have to get a speed reference from the t-case. If you do not, the speedo will read twice as fast when you put the case in low range. But you will be in 4 low so who cares how fast you are going. As mentioned the steering box will have to be relocated, and the truck will require some lift. That engine crossmember has limited clearance so you will be looking at some lift to even clear it. The other option would be to remove it and replace it with a tubular unit."

The best advice given. X100

Another option would be to put the body of the 78 onto the frame of the 86.

The cleanest way to make your 78 a 4x4 would be to sell the 86 and buy a 77-79 f100/150 4x4 frame and running gear.

Or one already 4wd.