For the transfer case we opted to go with a 3.8 Atlas II. I thought about using an OEM case but it looked like the available ratios were 2.71 or 2.69, and with the low gear in the 4R100 tranny at 2.71 coupled with 4.10 diff gears, this gives you crawl ratios of 30.1:1 and 29.9:1, respectively. I'm spoiled in my current truck with 96:1, so I wanted something better than what OEM offered, especially when this van starts gaining weight. With the Atlas, my final ratio is 42.2:1. And I must admit, the selectable front/rear drive is pretty cool, although who knows when I'll need it (but I've said that about a lot of stuff).
I spent a lot of time researching forums trying to figure out whether the Atlas would even package well with the 4R100 and couldn't find a definitive answer. I heard both yes's and no's. I know SMB made a few 7.3s with the 4x4 conversion but didn't know whether they used a special adapter. I decided to do it anyway and just make it work somehow. After ordering it, my wife and I made a visit to SMB in Fresno and were talking to their 4x4 guy, Heber, and he told me they may still have the special adapter they used that allowed them to mount the t-case without removing the stock 2wd output shaft or even dropping the tranny at all. It sounded too good to be true! He ended up digging one out of their attic and I bought it.
It turns out it really was too good to be true. The adapter basically mounts to the back of the tranny in place of the 2wd tailhousing and takes up the length of the shaft and then accepts the Atlas on the back end. Once I actually crawled under the van and mocked everything up, I realized this adapter put the Atlas so far back that I would have to relocate the entire front hanger for the gas tank. This also meant shortening the tank much more than you would with a standard 4wd tailhousing and output shaft. So consequently, I scrapped the SMB adapter and went the hard route and dropped the entire tranny. Since you have to tear apart the entire transmission to remove the output shaft on the 4R100 we opted to do a standard rebuild and beef-up of the trans even though it only has ~30k on it. I'd rather not have to remove it again for a loooong time.
Trans back from the shop with 4wd output goodies:
For those interested, the output length of the shaft past the tailhousing is about 1.25":
A side note: If you're going to do this, buy a transmission jack from Harbor Freight or something. Using 2 floor jacks is extremely difficult since you need to pull the tranny straight back. I did it coming out but definitely did not want to do it going back in. You waste so much time adjusting, securing, aligning, adjusting, supporting, adjusting, aligning, re-securing, almost losing it, supporting, etc etc. Couple that with working on your back and it's definitely no fun and time consuming! The cheap one from HF is decent.
To mate the Atlas, you need a total spacer thickness of 1" from the output face of the Atlas to the mating face of the trans. The Atlas comes with a 3/4-inch adapter ring to provide more clocking options. You have 2 choices here: (1) Eliminate this adapter and go with a 1" thick spacer. This limits your clocking options to the higher degrees (15deg and higher, maybe?). (2) Keep the adapter and add a 1/2-in spacer. This gives you less spline engagement but still more than enough. You also keep all of the clocking options. I went the 2nd route and I'm glad. After heaving this thing up and twisting it around on the back of the trans for awhile, I found that I could run 7deg clocking, clear the floor pan, and still have an adequate front u-joint angle.
By the way, I'm running a 1350 CV in the front and a 1410 non-CV in the rear (or at least that's the plan). Running a CV at the t-case requires the output pinion axis of the axle to ideally point directly at the output flange of the t-case. Clocking the t-case at 7deg doesn't give you this, but it's not too bad. I've done this on other builds and not had any problem. Problems would arise if the angle gets much higher than 20 degrees at high speeds, which this will never see. Shifter modifications and installation will come later.
2003 Ford E-350 PSD 4x4- conversion in process
1994 Toyota Xtracab Pickup- 22RE, dual cases, bobbed bed, homebrewed SAS, bumpers and rock rails