The biggest problem with the ND is the gearbox. It's optimized for efficiency and weight - maybe a little too much. Hard driving can take it apart, especially when high power is involved. The problems can be mitigated by running a transmission cooler to give your transmission the best possible chance, but that's not enough to make the car truly bulletproof.
BBR to the rescue! This kit will allow you to install an NC transmission in your ND. Now you have enough strength to deal with turbo power and without giving up the feel of a Miata shifter. This is the same basic gearbox used in the Fiat 124 and of course, a decade of NC production.
The transmission bolts up to the stock ND Power Plant Frame and keeps the existing driveshaft. You will need to make a small modification to the bellhousing to clear the ND starter and put a small dent in the tunnel to clear the clutch fork. You will also need an NC 6-speed clutch and (of course) a transmission. Note that the 2006 6-speeds would sometimes break a shift fork under heavy track use, so we recommend a later one.
Note that the rear end ratio of the ND 6-speed is an unusually high 2.88 to match the gearing of the stock transmission. You will want to change out the final drive to one from an automatic ND or a Fiat 124. The latter is available with an LSD as a bolt-in if you can find one.
The kit includes an adapter plate, a new flywheel, dust cover, and fasteners. Be aware that an ND transmission removal is a little tricky and there will be some problem solving required, so we recommend installation of these parts be performed by experienced mechanics.This kit doesn't include step-by-step instructions.