I would think that since the driveshafts in these cars are in a fixed location, it wouldn't be that hard to custom up a couple of driveshafts and stick your 2 speed case in the middle somewhere - maybe put two bucket seats in the back and let it stick up the center since the gas tank is under the ds.
I haven't actually seen one, but I understand it hangs off the tail of the transmission, and doesn't take up a ton of room since it just has a straight through or a .5 to 1 (guess) ratio option. So a shortened driveshaft would be in order. I'll do some reading up and find out what all would be needed, start saving up for it maybe. I'd really rather do a six speed manual swap, or even a T-45, but that's lots of money and fabrication and I don't have the time for that currently.
Ok, was digging around on their site...
With 4.10's, and their tailshaft OD which uses a planetary gear, you get a final drive ratio in 4th over of 2.27 to 1 which is a slight improvement over the stock in my case 3.27 to 1 rear ratio which in OD has a final drive of 2.32:1, so it seems like a best of both worlds situation... you can leave it in automatic mode and it'll split every gear giving you an 8 speed, or you can turn it off and run your regular 4.10's on the street or strip, and kick in the double over on the highway for some improved mileage.
I'll have to look into it, but it's a compact unit, goes in place of the stock tailshaft, you use a shortened driveshaft which on the thunderbird HAS to be an improvement, and you get better acceleration, and better highway mileage.... In theory anyway.
Any ideas, anyone ever used one? Anyone?