I'm certain High is 1 - 1, and Low is almost 2 - 1, giving a reduction of 50% in speed (or in your case doubling of speed). Just 2 pairs of gears with a mechanical selection. You also have selectable output drive for front axle, and on some a full-power pto drive.
They will cope happily with 130 hp, but I don't think 250hp would do them a lot of good and they certainly would not survive a reversal of drive to act as a speed INCREASER with that available torque :shake:. TM boxes are better-sized but very scarce.
MJ transfer boxes are not like the heavy old Militant boxes which I believe have been successfully reversed.
Also, the Tatra gearbox would be running at up to twice the speed, you could be leaving yourself wide open to a claim gearbox abuse :cool2: I like your way of thinking though!