I have a '81 4X4 F-250 4 speed with the 400 in it and I was wondering if anybody possibly knew if swapping to a 5 speed transmiission would require a ton of suspension or rear end modification in order to make it work? please please.

 

The only 5 speed you are going to be happy with is a zf. Problem is, the bellhousing is made onto the zf tranny. Lucky you, your 400 has the same bellhousing as a 460, which did come from the factory with zf 5 speeds. The other problem you may run into is all of them are later model and use hydraulic clutch systems. The bellhousing is setup for a slave cylinder system. So you will have to convert your mech linkage to hydraulic, which will require more parts. I think it can be done, but it won't be cheap.

Instead of all that, if you can come up with $3000, you could buy a overdrive unit that will bolt to the rear of your transfer case. All you will need to do is shorten the driveshaft. This would give you a nice overdrive in 2wd.

 

yeah Gear Venders, Argggg Drooool. I wish I had the scrap for one of those.http://www.gearvendors.com/4x4.html#ford

 

Bummer. It's what I was already assuming though so not too big of a surprise. For $3K I'd just go pick up a new F-250 from the auction. In ya'lls opinion what truck from the late 80s (model & engine /tranny combo) makes for a user friendly, but strong truck for just towing stuff on tow dollies around town?

 

I would find a 90-94 1/2 7.3 diesel idi with a zf 5speed. Those engines love to pull, and are fairly simple to work on.

 

Changing transmissions in a Ford truck is one of the easiest swaps I've ever done. I swapped my last F-250 from a C6 to a ZF5, this was small block stuff mind you. That truck paid it's dues and after much use and abuse I took the driveline out and various other parts and sent the rest to scrap.

I will soon be putting the ZF5 in my current F-250 (another small block '86). It's very easy swap. You must move the trans crossmemeber back a couple inches to properly mount the ZF5 (longer). You will need to modify the trans shifter cover or find one from a later truck that had a ZF5. Front driveshaft needs to be longer (hard to find, but approx 48.5" IIRC, I can measure if you want). Rear driveshaft needs to be shorter, I forget the length. I couldn't find a rear shaft for a ZF5 regular cab truck so I had mine shortened. Cost about $60 and had the shaft the next day. For the clutch, just use the stock ZF5 master and slave cylinders and make the appropriate holes in your firewall to get it lined up with your clutch pedal. IIRC there may be a template on the firewall of the mid '80s trucks that used hydraulic with an external slave cylinder on the NP435, as opposed to a mechanical linkage.

One thing to check for your 400 would be to measure bellhousing depth fo your 351M/400 NP435 bellhousing and compare to the depth of a 460 ZF5. Simply to ensure the snout of hte ZF5 will properly sit in your pilot bearing if it you will need to space out either the pilot bearing or the bellhousing.
All in all the swap is easy as pie for a small block and I don't imagine too much difficulty for a 351M/400.

 

When I did my ZF swap I was able to reuse my original front drive shaft without having it lengthend. I used the rear shaft from my donor truck since I swapped the t-case as well and the outputs were different. The donor truck also used larger u-joints which was cool.