Mk Mark Walker (flat track on here) has a sequential for sale I think. I used the oppliger long box, close ratio on mine, it is not too noisey and makes the car much better on the road with 3.9 riffs as the standard cars were over geared.

What torque is yours making, at 580 bhp I would expect a fair chunk of torque too, most break around 400ftlb.

Bara boxes are used a lot, as are fixit.