Any ideas where I could get a 6n gearbox strengthened or gearkits?

I've already tried quaife.

My second gearbox won't be lasting long.

What do you mean? Heavy duty bearings and bolted diff are the only thing that comes to mind. To be honest, new SKF/Timken bearings are a good start.

Don't know of higher quality gearsets, if they exist I hope you have a ton of money to spend.

Edited 16 February, 201610 yr by Vic

Vic is right about good quality bearings and seals. Talk to a company like BRT bearings about good quality, heavier duty components. Micro polishing all the stressed items in the gearbox will make it less likely that the parts will break. I got a price from a company that makes racing motorbike gearboxes and they charge £10 per item for micropolishing. Parts are put into a vibrating drum along with ceramic chips and left for a long time. This removes stress risers from the parts which effectively strengthens them. If you don't want to destroy the gearbox by spinning up one wheel, Quaife is the only option (unless you find something used from other manufacturers) but as you probably already know the final drive gears have to be the bolted type which are only on certain gearboxes and on line in one ratio. The other thing I will look at will be fabricating possible bearing supports which will strengthen the bearings that carry the main two shafts of the gearbox.

As Vic says, you won't do any better than SKF or Timken. Have a talk to your local bearing stockist.

Regards