hillman Imp gearbox question

Just about - but it's more dependant on torque and in particular shock loadings.

Road-going Imps have run up to ~100bhp on a stock box without trouble, but cars run on rallies can kill them at ~50bhp. The difference is that on a rally you might have one wheel spinning on mud which suddenly grips as it hits tarmac - a big shock loading. Dumping the clutch at high rpm can break them too.

Imp boxes tend to break around the diff under shock loads - the casing splits aorund the output shafts or the diff pin breaks. If you are using solid driveshafts rather than rotoflex couplings you need the sprung-centre clutch plate from a Minor (the Imp's one is solid so there's no cush drive between the engine and box).

The other thing that can kill them is the fact that the oil tends to get pumped to one end of the box at continuous high speed (~70mph). Regularly slowing to 50 re-distributes the oil. In addition they can start to get hot above a sustined 70mph - the ideal solution to both problems is an oil circulation pump and an oil cooler.

Finally, bear in mind that the gearing is not ideal for many engines - on 155/80 R12 tyres top gear is 15mph per 1000rpm, so 60mph is 4,000rpm.

What engine are you planning on using? And is it going to be in an Imp or something else?