yeah this would be an extensive swap , and you are probably better off with a vg30et , but if you want to be uniqueand do something different go with an rb ( preferably an rb25det ) its a better engine dohc too , and there is alot of more parts available for that engine.
you would have the same proibs fitting an rb into your z as you would putting in a vg30dett .
if you want to make life easier on yourself just stick to the vg30et
the z31 n/a tranny is decent. its stronger than the turbo tranny that they used from 84 to 86. it should hold 300hp no problem. plenty of people have 240sx's with 400+hp and the same tranny and it hasnt blown up yet.
neither the transmission nor the driveshaft will bolt on, you need to make a custom driveshaft and use the FS5R30A trans out of the Z32, and no, when you make your custom driveshaft, you can make it as long as it needs to be to make the two work.
easiest way: get them your car so they can see what they need to do.
if thats not possible, they'd need to know exactly how long the shaft needs to be, what kind of power it needs to hold, what kinds of joints it should have, and how it mates up to the diff and the tranny.