You're exactly right.
The Cab and Chassis uses 2.5" wide leaf springs, a total of 14 on each side of the truck.
If someone was planning on converting a cab and chassis over to a SRW rig they would have two options.
A-Weld 2.5" wide spring erches on the SRW axle. Or...
B-Cut the 2.5" wide brackets off the frame and mount 3" wide pickup brackets. I don't know if the existing holes line up, so don't rely on this route because you may end up drilling a bunch of holes and find it a waste of time since you drilled away a bunch of good metal that would support the weight...I don't really know, just a brainstorm!

The advantage to pickup springs is the ride quality.
Pickups have fewer leaves than a cab and chassis, (5 I think?).
The fewer springs provide more flex and cushion.
Overload or Helper springs on a pickup do not effect ride quality because the helpers only get action when the load eventually sags the truck enough to hit them.
You can get creative with pickup leaf springs by keeping the regular spring pack, then adding a hefty helper pack. This will give a nice ride all the time, then still be able to support a lot of weight with the helpers.

 

What I would like to do is convert my '83 351W powered cab n chassis F-350 to a 4x4 SFA trailer towing/pulling rig. I'm not concerned with ride quality too much as I wont be driving it for long distances. From what I understood, my best choices (or the ones I find the most intriguing at least) would be for a Dana 60 front axle from a 86-97 one ton (I realize I may have to buy different hubs if it's not a DRW donor truck) a NP435 or a T-18 4 speed trans, and a NP205 t-case (heavy cast iron is always a turn on to me...lol). As far as the NP435 and T-18 transmissions are concerned is one better than the other, or are they on a pretty level playing field? I'm also wondering about this doubler thing I've been seeing (with two t-cases) I've noticed people have made them in different ways. Buying an adapter, fabbing a plate and get rid of the rear half of the front t-case, and probly the cheapest of them all removing the tail cone of the front case and home building an adapter to bolt it to the rear t-case. What I was wondering about these are what Ford style t-cases can be adapted to work like this? Thanks for reading, I'm sure you all are tired of reading my ramblings by now.

 

Does SFA mean Solid Front Axle?
Dana 60 front from 86.5-97 F350 is the only solid axle that will fit your truck without major modifitions. I can't recall what year the King Pins stopped. '91 I think? Anything after that switch-over year will be ball joint. Ball joints are not as strong as king pins, but you probably won't notice the difference. They might just need to be replaced sooner is all.
If the donor is Single Rear Wheel, you'll need the Dual Rear Wheel hubs so your tires match front and rear.
NP435 and T18 are both very strong 4 speeds and you likely won't see the weak points of either. It "could" be argued that the NP435 is weaker because the top cover is aluminum and it has oiling issues while flat towing it.
You're right about the doubler cases. But I doubt you'll need to get that low. The NP435 behind 352 FE has that killer 6.8 low that it is slow enough to pull any trailer I hook up to.
Multply the NP435 low x 2:1 low of the NP205 and you're at 13.6.
Multiply the NP435 low by 2.6 of the BW1345/1356 and you've got 17.7
Common doubler is the NP203/NP205 doubler, you'll find more info about that with a google search. Pirate4x4 is where all that crazy stuff happens.
The best info I've found on this topic is the Klune Goliath Underdrive. It bolts inbetween the transmission and transfer case and reduces the gears x 2.7 before entering the transfer case.
I'm VERY interested in this as well. In my situation, I'm trying to get two gear boxes behind my main transmission. The first will be Overdrive and Direct gears for gear splitting and the rear box will be a NP205 transfer case for 2 wheel drive LOW when starting a heay load, and for 4x4 to the front wheels. I've spent a LOT of time on this topic. Researching products, building Excel spreadsheets to show ratios, and I'm starting to collect parts to see if I can even possibly convert the NP203 front case of a doubler into an Overdrive splitter. I'd keep the direct gear, but ditch the 2:1 low in favor of a .78 OD ratio. I've never opened up a transfer case so I'm not sure how the gears mesh. I'm sure I could design a gear to work, and have a machine shop build the gear for me. It wouldn't be a synchronized shift, but I would sure learn QUICK!
Good Luck!

 

Yes, SFA does indeed mean solid front axle, IFS is independent front suspension(my opinion ---> IFS= ick.. I hate IFS rigs).

You have a very interesting plan with the transfercase overdrive thing. I dont see why it wouldn't work. You just may trouble coming up with a set of gears off the shelf you may have to get them custom made. Of course I would've realized you allready knew this if I read your whole post... lol oops! Great idea!

 

I spent some time looking at the front spring hanger location on a F250 4x4 and my frame F350 2wd. I noticed a couple of things. The front hanger bracket for the 4x4 is on with 6 rivets, 3 of which mount the core support brackets also. These three holes are on both frames and can be used to locate the front bracket on the 350 frame. I also noticed the center bolt that goes through the hanger bracket. The 250 4x4 frame has a dimple on the outside of the frame for this bolt to sit it, the dimple in also locates the spring shackle. The 350 frame does not have this dimple, is a longer bolt and spacer needed or does this section of a donor frame need to be spliced in?
JWR

 

I'm not sure but it seems to me that one would need that. That is if you're talking about what I'm picturing. I'm still trying to round up the parts for my truck, axles, t-case(s), trannie, and driveshafts and such. Does anyone know a good site to find any of these?

 

He asked last January and has prolly figured it out by now.

Junk yards, Craigs List, broncograveyard.com, blueovaltruckparts.com, NPD, LMC, etc.