I have to say that I admire your intrepid spirit. While rebuilding a BW 1339 Quadra Trac some 20 years ago, I became intrigued with using the low range box as an underdrive. Its planetary setup seems plenty stout enough, and while it's not exactly a dainty unit, compared to an NP203/205 doubler it seems compact and light weight indeed. I did look at the Model 18, but after eyeballing it, I figured it was too small. I decided that a Land Rover LT230 would be just about ideal. It seemed closer to the right size, and I really enjoyed the convenience of full-time 4wd for the type of driving I did. I had a couple complete QT cases with low range taking up space in the garage. All I needed was some LR PTO gears to fool around with, oh and the LT230. Well it turned out that the gears came in several iterations, none of which are readily available in the US. And if you did happen to find an LT230 in a salvage yard, they weren't exactly giving them away. I basically abandoned the idea after a few phone calls. You have much more tenacity than I, and I think the Model 18 is the only choice for a flatty. If I remember right, I think the LT230 is at least 4" longer than the Model 18 and chunkier in every other dimension, too big and too English for your application. I am very interested to see how this works out. Subscribed.