Ordinarily I'd say Steve Butchard from BMS as he makes fully manualised, normal (forward) pattern, NO ELECTRONICS modifications to JATCO's. He makes a transbraked valvebody for them that is the shiznit.
I don't know how seriously you're going to want to drag race the car so unless you plan on putting slicks on it and turning into a rotary drag nut, I wouldn't bother with the transbrake.
You should make a call to MV Automatics in S.A. and speak to him about the manual valvebody he does for the JATCO's. If it's the 300ZX box, it's an RE403A. Exactly the same valvebody as a Skyline RE401A and in fact, most of the parts are interchangeable. (403 will handle more torque than a Skyline/WRX 401A but the larger components absorb more power. A reasonable trade-off in the circumstances.)
If Mike at MV can build you a box that is:
A/ Completely manualised
B/ Fitted out with tough steels, bands, clutches, etc
C/ Refurbished and re-bearinged (is that even a word?)
D/ Equipped with a converter of his manufacture
...then I'd say go with him for the box. He certainly knows his shit when it comes to JATCO's and although there isn't a transbrake option with his modified valvebody (i don't believe anyway) and the over drive (4th) gear will be a function of an electronic switch (possibly like it is now) it is almost certainly the best way to go about strengthening your box to handle what you're about to feed it.
Cheers,
Adrian