I can't quote on the strength of the 350z box, but what I do know is that most s15 performance tuners swap there 6 speed gearboxes for 5 speeds because of strength issues, the S13/S14 5 speed gearbox is nearly bullet proof
On the swap, well gearboxes can be picked up from R2000 at importers there condition is well questionable, the body already have all the holes and necessary mounting bolts needed to convert to manual, where you gonna find a pedal and its bracket that is the question, other than that its getting a hydraulic master and slave cylinder AM parts widely available as its the same as the 1 tonner bakkie and the rest is peanuts... If the box doesn't come with a shifter you can pickup a AM shifter from Rezlo
On the crank of the engine there is a extra fitting that needs to be removed (some spacer thing for the torque converter) found out the hard way...
Then you might need to bypass a wire on the ECU to fool it in being a manual... this is in the S13 not sure about S14...
Don't forget you'll need a new clutch as well other than the Valeo crap available at parts store which doesn't hold if you do slightly hard driving, speak to Rezlo for a clutch.