A manual transmission swap is fairly involved. I apologize but I don't have enough time to get into it. There are some good articles available, they should be searchable with google or another search engine.
Do a Google search on this (1996 Nissan 300zx repair manual CD) This is your repair manual that can be downloaded and the Black Dragon link on this page is one of the best sources for parts of all kinds. TomballPI
There are two plugs on the 300zx manual transmission. on on the passenger side and one on the bottom of the transmission. You can unscrew them with a 1/2 drive ratched. fill through the plug on the side, drain from the plug on the bottom
Don't do it. It's a sports car, learn to drive a stick. If you are replacing an automatic transmission... then I would think this would be a good opportunity to upgrade to a manual. You're cutting off about 20hp and 10ft lbs of torque by keeping an automatic transmission.
There are several things that can cause your 1986 Nissan 300 automatic transmission to send you a start leaking fluid the most common cause is a blown seal in the front of the transmission. There is a seal in the rear of the transmission. The transmission pan gasket can be leaking.
Now this depends on whether your car is a manual or an automatic and a twin turbo or a naturally aspirated version. Go to http://300zx-twinturbo.com/cgi-bin/manual.cgi There it will show you how to replace it.