I looked all over for how to swap in a NP205, and found some useful info, but it was all over the place and I wanted to consolidate it into one spot. Plus I think I found a pretty good solution for a problem that occurs. Anyway, here goes.
The Dana 21 uses a "2wd" NP435. The 2wd NP435 has a square bolt pattern on the back, and uses a gear that that is held on by a nut to the back of the output shaft to interface with the Dana transfer case.
The NP205 uses a "4wd" NP435, which has a trapezoid shaped bolt pattern, and just uses a splined output shaft to go directly into the NP205.
So, in order to do this swap, you'll need
"4wd" Np435
NP205
435 to 205 adapter (married case)
1980 (and later, not sure where it ends, at least until '87 I think) transmission mount
Flat stock steel, at least 6"x8", 1/4" thickness would be fastest.
rear driveshaft (for sure)
front driveshaft (good idea)
All linkage associated with the NP205
NP205 rear bracket
As you can see from the list, the best way to do this is to get a later model donor truck. I found a '77 F250, with long bed and manual trans and took everything from the bell housing back, including the both driveshafts.
The 4wd NP435 will bolt right up to the old bell housing. No need to change (or obtain) a bell or any of the NP435 clutch linkage. Good time to change the clutch if you need it. Ours works fine, and we need the truck back on the road, so I didn't. I may regret this later.
I did have one issue installing the NP435 back in the truck. The NP205 adapter is too tall to have it bolted to the back of the transmission while you're installing it. The adapter hit the floorboard before the transmission was high enough to line up the input shaft with the throwout bearing and clutch. Not sure if this works or not on the later trucks, but it didn't work on mine. So the transmission had to go in by itself. Once the transmission is in place the t-case adapter fits on the back, no problem. You may want to secure the output shaft bearing while you're installing the transmission. Its retained only by the t-case adapter, so without the adapter the output shaft can slide back far enough to separate the input and output shafts and drop out the roller bearings in the back of the input shaft. Ask me how I know! If this happens you have to remove the transmission, fix the gears, and start over. Not fun.
Once the adapter is on, you'll find that the original transmission mount is too tall to fit between the transmission and the cross-member (the NP205 t-case adapter has a larger diameter than the D21 adapter).The mount location is also slightly further back than it was with the 435/D21.On our truck (F100) the cross-member is riveted into the frame, and also locates the trailing arms for the front axle, so its not moving. Looking at the later model cross-member (which I also took), it wouldn't help anyway, its not any deeper. After trying to figure out how to modify the transmission mount, I looked at a bunch of different Ford mounts over the years. Turns out a 1980 (and up, for awhile) F150 mount is exactly the ticket. I bought the mount and made absolutely zero changes to it. Which means if you ever need a new mount, you can just buy one and bolt it in. I then fabricated a flat plate to bolt the mount to, the assembly then bolts to the cross-member just like the original. I used 1/8" plate because its what I had (same thickness and the stock cross-member bracket) and reinforced it since the original mount has structure and is therefore stronger. I doubled up the plate where it bolts to the cross-member, and then tied it all together across the top with a doubled up section for the transmission mount. End result, the plate is 3/8 of an inch tall. If you had 1/4" stock you could just skip all this and just drill the holes, it should be plenty strong and tall enough to get the engine/transmission in the right place. I had 1/8" and a welder, so there you go. I drilled 4 holes to mount it to the original holes on the cross-member, and then 2 more for the new transmission mount. This also takes care of the mount location being further back.
Original Bracket
New fabricated plate
With the plate and the new transmission mount, the transmission/adapter/NP205 now bolt right in.
Taa-Daa!
This is where I'm at now. As for the driveshafts, I'll say this. The "new" front driveshaft is almost exactly the same length as the original. It shouldn't be a problem to use the original if you can't find a complete donor. You may have to get "adapter" u joints though, the kind with 2 different widths. We already have a '74 Dana 44 in our truck, so I don't know for sure all the different yokes that were used.
The rear driveshaft may be an issue. Our '71 F100 is a long bed, and we have a 2 part rear driveshaft, its definitely too long (at least using the standard mounting location for the driveshaft strap). It looks like the "new" driveshaft (also out of a long bed) may be a touch short, the slip joint is pretty much maxed out on a trial fit. But this may have everything to do with the 4" lift on our truck. I like the single rear driveshaft idea, so it may get lengthened. I'll update when we get the driveshafts installed. I also still have to install the rear bracket on the NP205, I got this from the donor truck though so it should be as simple as drilling the frame for the bracket. We will have to reroute the driver's exhaust though I think.
