The interesting things is if you move the engine forward, you'd probably have room for a Marlin Crawler. The Doubler adds 6" like the Klune V. With two 4.70:1's you'd have close to 500:1 with 5.13's. And with 3 you'd have 2000:1
I'm not sure if the Toyota transfer case uses a similar 6 bolt pattern to the Dodge/Jeep. BUT from what I was reading the W56 and W59 are almost identical to the AX-5. So at most you'd need to swap the bell housing. Not sure about the input shaft length but as long as it's not longer then the Jeeps, it could be fixed without swapping that.
The other interesting thing is Marlin sells rebuilds that are supposedly HD then stock. They're in Fresno so it's not that far. Makes you wonder about doing a Marlin Crawler. Something to think about especially if they can build some beef into the transmission.
From what I understand the W56 will swap in behind the 2.5. Then you can run the Toy boxes. You would need to swap the frontend over to pass. drop.
I would guess the 511:1 has 2 4.7 boxes, which is kinda dumb. It's a much more user friendly to have various ratio's in each box.
I know Marlin has 3 boxes on that truck. He did it to prove it could be done. IIRC it has 2 4.7's and a 2.28. But I could be wrong on the ratio's.
Having super low ratio's is all well and good but I have not been in a situation where I wished I had lower gearing. (other than a turtle race) Trail type wheeling I rarely use Lo-Lo 1st. I can't stop it with the clutch out even on dry pavement in 2 lo-lo. I've had the front tires skidding on black top and the idle barely dropped.
If I worked for Marlin or was family I would run his gear for sure. The Toyota stuff is super strong! The gears in those boxes are very large. Plus Marlin is a super nice guy!