I did this exact swap on my 89. I bought a 1989 wrecked donor.
The c6 driveshaft would have worked. But I picked up the donor driveshaft and it was 3 times as heavy as the c6 driveshaft and it had larger u-joints. So I decided to take and swap yokes on the rearend and use the heavier driveshaft from the donor. But like I said I tried it, the original c6 driveshaft would have worked, it was just a little bit longer than the zf shaft.
The bottom transmission crossmember was exactly the same between the c6 and the zf, I held them up side by side. But to use it with the zf I had to move it back on the frame 1 inch. My 89 frame had the holes drilled already. But the upper crossmember pieces on each side for the c6 would not work. No extra holes on the top part of the frame. I got the upper zf crossmember pieces, they have more of a offset to them and they fit perfectly in the original two upper c6 holes in the frame.
The wiring was plug and play. There is a large electrical plug under the brake booster area. The most aggravating part of this was getting the wiring strap loose on top of the frame that holds this harness. Once I got that off on each truck, the zf harness plugged right into the large plug, and plugged into the reverse switch and the transfer case switch. WARNING; Happens everytime people do this swap, they get it all together and the truck will not crank. That is because your c6 had a neutral safety switch and wiring in this harness, the zf does not have a neutral safety. But the zf harness I plugged in has a jumper made into the harness just for this purpose. If you do not have the factory zf harness, you will have to make a jumper for the two red/blue wires that went to the c6 neutral safety.
You need the floor cover from the zf truck. It is higher to have more clearance for the zf. Of course I just swapped this out, it's easier if you take the seat out. If you do this first,, the upper bellhousing bolts will be staring right at you through the hole in the floorboard.
You will need to swap pedals. Ford in their wisdom ran the speedometer cable right through a hole in the pedal assembly, so you have to pull the cluster and disconnect the speedometer cable. Just another aggravation to add to the list. My truck had a little cover plate where the clutch master mounts, I just took this out and bolted in the master from the other truck. I think some of the earlier trucks are not already drilled, but have dimples marking the location where you need to drill the firewall.
The steering column; I elected not to swap steering columns. I took the shifter lever off, but I had to take a piece of wire and tie up the lever at the bottom of the column to hold the old auto shifter mechanism up in park position. If you do not do this, it will fall down and then when you turn the truck off, it will not let you turn it back far enough to get the key out, it thinks the truck is still in gear.
