This is my latest project...getting rid of the (weak) Model 20 transfer case and the (antiquated) SM420 shown here in favor of a five speed NV4500 transmission and a new STaK transfer case.  The trans should improve drive-ability and the STaK case will give me an extra super low 5.4:1 range in addition to the ranges I am used to having.  My low speed crawl ratio with the new setup?...182:1.

The first step was to study my options.  I considered other trans and t-case options but chose the NV4500 for a combination of a heavy duty manual trans (thought about auto, but I guess  am just old school) and modern synchros with an overdrive gear.  The STaK is a bit of a risk as it is unproven compared to say, an Atlas.  I feel that it is justified based on the extra low crawl gear and the apparent quality of the unit.  I know Advance is coming out with a 4 speed case, but their design is essentially an Atlas II with a HD NP241 multiplier planetary riding on the front.  This creates a situation where even if you choose the (lowest available) 3.8 internal gear, you are multiplying it by 2.72:1 when you choose low range, giving you an effective 10.3:1 low-low range.  I think this is too low and leaves too much of a gap between low and low-low.  Also, they aren't in production yet, and I want to get going!

After contacting both Advance and Novak, I decided to go with the following combination of adapters and parts:

-- Advance NV4500 to GM small block bellhousing kit (71-2577) includes bell housing, pilot bushing, dust cover, pivot arm, and the rest of the little parts ($280 from Rock Buggy Supply)
-- Advance NV4500 to Dana 300 tailshaft housing (50-0205)
-- 11" Centerforce pressure plate and disc ($180 from Advance)
-- STaK transfer case with 23 spline input gear and 1310 CV yokes ($3010 from STaK)
-- NV4500 from Transmission Exchange in Portland, Oregon with the following features ($2310)
- Dodge 23 spline output shaft
- Chevrolet input shaft to match the GM trans and bellhousing
- 6.32 early chevrolet first gear
- synchro reverse gear

As of January 10, 2006, all of these parts have been ordered.  As they come in I will chronicle the project and put up some pics.

January 12, 2006 --- DON'T MAKE MY MISTAKE!!! I originally ordered the Advance 712576 Bellhousing kit (shown on the left).  It arrived, I opened it up and posted a picture.  Fast forward to February 1st, 2006...tried to match up the bellhousing to the trans and they don't fit.   Started doing some research and realized I had ordered the wrong the first page of the Advance NV4500 conversion manual they show both patterns as indicated in the pics below.  I really needed the 712577.  Learn from my error and look at these pics and diagrams and do some measuring of your trans before you arder
NV4500 and STaK Transfer Case Conversion Page
Also got the clutch disc today, a Centerforce 383735.  I elected to re-use my pressure plate as it is not that old and should be in good shape.  That may change when I pull it apart and look at it.
Total parts list (edited as I go).
January 16, 2006 --- Haven't had a chance to work on the Jeep for a few days, but had a few hours today, so I started tearing the old parts out of the Jeep.
Out with the old, in with the new (soon!)...this is the SM420 trans with scatter shield/bellhousing, the Dana 18 transfer case (in a 20 case), and the Warn overdrive unit.  When I'm done with the project (and sure I don't need any incidental parts), I'll sell them for wahatever I can get and try to recoup some costs.
January 17, 2006 --- Got the NV4500 today via freight.  Here it s still strapped to the pallet in the back of my pickup:
After receiving the trans, I started doing some measuring.  First, I measured the physical size of the trani...obviously it is longer and larger than the SM420.  Its rough measurements are 11" wide by 12 1/2" tall by 19" long.  The "stickout" (how far the input shaft protrudes from the face of the trans) is 6 1/2" and the pilot shaft is .590" in diameter, matching the pilot bearing.  This doesn't really tell me whether or not it will fit in the Jeep, but does give me an idea if it is even possible.  It's gonna be close without a body lift to get the sheet metal away from the top of the transmisson and tansfer case.
This is the next fit test.  With the help of a friend, we attempted to mate a stock Dana 300 (a spare) to the output of the NV4500.  As you can see from the picture, it mates up, almost.  The splines engage and the bolt pattern is the same, but this was as far as we could get it to engage.  The problem can be seen in the next photo.
This is the other half.  The front retainer on the Dana 300 protrudes from the face of the transfer case 1 3/8".  Obviously the Dodge tailshaft housing isn't deep enough to insert the transfer case completely. 
Here is half of the problem.  The depth of the tailshaft housing on the NV4500 is about 3/4".
Hopefully this is the solution, Advance Adapters 51-0205 tailshaft housing designed to adapt the NV4500 to the Dana 300. 
As you can see, the depth is about 1 1/2".  This should be just about enough room for the bearing retainer on the STaK.  After calling STaK, the rest of the dimensions will be very close as well, but I think it will work.  The only way to know for sure is to bolt it up.  STaK says my t-case should ship the middle of next week, so I should see it in a week and a half or so.
Since this page is getting long and cumbersome to load, I've divided it up.  To see more go to the

*Important note - the tailhousing from AA didn't come with a seal.  After some measuring, I installed a CR 19211.  It should work well.
Total parts list (edited as I go).
This diagram is the most common NV4500 transmission face pattern and a pic of the corresponding Advance 712576 bellhousing above. As indicated by the title of the diagram (stolen from Advance..thanks!) it was used on the late GM NV4500's and all of the Dodge NV4500's.
This diagram is the less common face pattern and  pic of the corresponding Advance 712577 bellhousing above. As indicated by the title of this diagram it was used on the early  GM NV4500's.  This is a more desireable trans becuase it has a lower first gear (6.34:1 instead of 5.61:1), but it has to be ordered specifically for the application. This is what I ended up using.