The miata transmission is very nice, but can't handle safely more than 250 WHP.
At stock situations or at light turbo setups you 'll not deal with problems, but with 300+ WHP the 2th or 3th gear sometimes will gone
So there are two solutions
1)exotic stuff gears (os giken, quaife etc) but very expensive
2)adaptate transmission from another car, more inexpensive from the first solution but very difficult to do a perfect job
So I put a turbo trans model "R" from RX7 fd3s (93-95).
It was from the beginning a difficult modification but the engineer succeed in this 100%.
The gear lever is exactly in the same position like the stock, and the "sense" of changing gears maybe is better than a stock transmission miata.
The 1-2-3d gears are a little shorter than stock gear ratio
The 4th is the same 1:1 ratio
And the 5th is much longer than stock ratio!! And this is very nice for a 340+ hp turbo miata!
The modification based to a new housing between the block and the RX7 transmission. We modificated the miata housing (bellhousing? ) and connect to its end, the end of the RX7 housing.It was a very difficult job because it had to be perfect at straight line with the stock one.
You will see at the fotos what exactly we have do because my english are not so good to explain exactly what I want to tell.
The driveshaft was cut and modificated to match to the RX7 transmission.