I wouldn't worry too much. I got a 5 speed box for Helens 410 and haven't even got round to fitting it yet.
We use it for the occasional long trip every now and again and it happily cruises along at 60 and easily pulls to 70 to overtake stuff.
The F10 is an amazing revvy little engine.
The highest bolt on gearing you will get is 5 speed box, Sammy diffs in 413 cases, 1300 transfer box, and the biggest tyres you can fit under the arches.
However you probably wont be able to pull away!!
Maybe someone could make a Road-lobster transfer box using the opposite halfs of the two gears.
