And back to the original question. Having worked as a tech for over 10 years, and having built and swapped drivelines on a number of different cars, the answer to your question is not really.
To do a complete auto to manual conversion, there are a number of different factors, and while I have not done one specifcally on a 240sx, I have done a few already.
Things to consider:
Pedal assembly.
Clutch controls (Master cylinder, lines, etc.)Slave cylinder ( dont remember what its mounted on a 240sx)
Driveshaft (length mostly)
Console, shifter assembly
Wiring harness, speedometer, computer controls and relays.
All of the above is going to need to be added or modified. I am not sure if the 240sx unibody was set up for both models, so you will have to see about that, and that is the biggest thing, because if you have to get into modifying the firewall for the clutch master, and underneath the dash for the pedal assembly, and get it all to match, it can be quite a challenge.
You also need to look into the computer for the transmission controls and relays. Its not like the old days where everything was manually operated. Everything has to sync, or you are gonna have check engine lites on all the time.
Again in the interior you will need to get the shifter assembly, and the correct console.
So what it all boils down to, is the 600 bucks for initial car cost, added to all the parts you are going to have to find, and pull and purchase (i wouldnt buy them new obviously) and everything you are going to need to do labor wise to convert vs. the cost of simply buying a comprable 5speed model that you dont have to do anything to, really worth it. My opinion is no.
I hope that helps, I realize your post was kind of old, and hopefully you hadnt made the purchase yet.