Well, as I recall (was a while ago now), the WH 435/203 was 23 spline - it was the 203 from the dodge automatic that I needed, so I thought all dodge's were 23 spline. I could be wrong, if I am, then someone on this board will probably correct me. I'm not 100% sure because all I did was take the new 435 shaft with me to the junk yard and stick it in 203s until I found one that fit. But I'm pretty sure it's 23 spline, because I kinda remember taking the same 435 output shaft and sticking it into my buddies Dana 300 as an experiment. And it fit.
See, the Wild Horse's kit is basically made up (this is my opinion, based on what I saw when I had it laid out on the floor) of a mix and match of AA parts. BOTH adapter plates had 'AA' stamped on them, as I recall. The shaft I am sure is the 435/300 shaft they sell. But AA does not sell this as a kit. WH buys the various little bits, then reassembles them into a new kit, and sells that. IMHO. The shaft they cut down a little so it wil fit between the 435 and 203 - I'm not sure what the 435/203 adapter was originally for, but I'm pretty sure it had AA stamped on it. So basically I think they figured out that if you took the AA 435/300 shaft, cut off 1.5 inches of spline, and used the adapter from some other kit, you could mate them together with a very thin (~1.75 inches, I think) adapter, then they used some other AA parts to adapt the back 1/2. Again, this is all conjecture on my part, that is just the way it looked when I had the parts laid out in front of me.
I am extremely happy with my setup. It works extremely well. For reference I run the stock 4.0 and 4.56 gears. Basically you can crawl as slow as you need to - no matter what angle you're trying to climb, it won't stall, no problem going slow on the down side - but at the same time it is really easy to get to upper ranges. Basically the great thing is the flexibility of the whole thing - no matter kind of trail I am on, I can always find a suitable range. There are other advantages too. For instance, backin up - the 435 has a super low reverse gear, if I had a single transfer case with 4:1 gears, backin up would get real painful, now I can pop the 203 into high real quick and get a much better range right away. helps when you are backing up long distances. I know you can always switch into 'high' with a single transfer case, but we are talking convenience here, folks.
Anyway, I am very happy with it and the way it performs. I got it within the confines of the stock skid, but it took some doing. And I'd like it to be flatter.
The other thing is worrying about the 20 strength. Yes, you can buy heavy duty output shaft stuff for it, but have you seen the price of that stuff???? Geez!
My basic plan is to some day go either atlas highlander or 205 (with 3:1 gear set in either the 203 or 205), and clock everything so it is higher. On my current setup, the 203 is 2:1, and the 20 is 2.46:1 - not THAT much difference, although you do notice it. The 'flexibility' of the whole thing would be increased by more difference in the 2 low ranges, so what you have set up sounds really really good.
So the basic plan is probably get the current 435/203 adapter machined to rotate the 203, then get either ORD or WMS adapters for either a 205 or atlas behind that...... whatever allows everything to rotate up flat.
So your 205 is also flat??
How much for the 3:1 203 gears? Sounds pricey.
WMS is expensive, but I've had nothing but good experiences TALKING to them (all I can do is talk to them, I can't afford to actually BUY anything from them yet) - talked to me for like an hour about 60 knuckles, even though he knew up front I wasn't going to buy his. That's service.