I have heard tons of feedback saying that swapping the Synchros out of a 5spd Pathfinder tranny is possible, and would be highly beneficial. (Not to mention cheaper than buying a new trans...) The pathfinder's transmission was built with the exact same spec synchros as the 300zx besides the fact that they are considered much more "heavy DUty" and are made of a much higher density material.
The only real problem with doing this is finding someone that is actually willing to rip apart the z32 tranny and be able to put it back together with the new synchros...
When the only tool you own is a hammer, every problem begins to resemble a nail. -Abraham Maslow