The 4.0 liter motor out of the new Frontiers would be a blast to have in a old Z. Stillen makes a supercharger that bumps hp up to around 400hp. Nissan Sport magazine has one on a Pathfinder that they speak highly of. Remember starting in 2005 all Pathfinders, Xterra's and Frontiers came with the 4.0 liter. In the Pathfinder it got an aditional 10 hp to 275. Also in the trucks and Xterra's there is a 6 speed manual available.
I have driven the 6 speed King cab trucks and they are a blast.