Track and rallye first gear is hardly needed but for street where most boxes end up its not for a light datsun but for a heavy celica. However I thinnk Simon you use the 2.9 first which is an awesome ratio to have.

71 series box sloppy fix also someone mentioned they used soft brass bushes?
Hard nylon sounds good specially for those rare and great 2.9 first 260z boxes.

T50 weigh is around 32kg as rallycar weighed it some years ago mine weighed 34kg
with dellow kit installed while 71 series stumpy box is 35kg and much stronger
with 3.25 first gear.

To change first gear on t50 its about 800-900 dollars from what Ive read.