Well it has to have 3 gears, or will spin backwards, so that makes it a lot longer. OR you could use a chain drive, but it would take quite a chain. 400lb-ft x 2.7 low range x 6.55 1st gear is about 7000. I guess that's the same as the 2nd case in a doubler.
How about reverse-cut ring/pinion and flip the whole axle over? Maybe an oil-slinger?
You could also make a rear axle out of a high-pinion front. Or just go to a Rockwell axle.