A gearbox can handle lots of power as long as you don't abuse it. I live in the land of big block V8s and most use automatics of course. But many live with Bog Warbler boxes for many years.

I lived with a T9 in my Merkur that had around 250bhp for many years. Adding big tires and a LSD makes the box work harder so now I have a T5 now - but I'm not hard on boxes.

Stay off gravel, change the fluid, let it warm up, double clutch downshifts and don't drop the clutch. Avoid a solid puck clutch too - use something with some springs in it.