you need, a 5 speed trans from any 240sx, any year, even the 84-86 300ZX N/a trans as well as I believe several other less popular applications fo the same transmission, you'll also need a datsun, L series transmission, any 71.5-83 is find with the exception of the 82-83 280ZX Turbo T5, anyhow...

you take the front case off both transmissions, it's pretty simple, you'll see the bolts holding the 2 halves together in the middle of the transmission, there are also some bolts in the bellhousing area that hold the cover over the mainshaft bearing and selector shafts, there si also a large snap ring here you'll have to pull, you'll need to take the front case, and countershaft bearing from each trans and swap them.......
to put the front case from the L series trans on you'll have to overbore the 5th/reverse selector shaft hole, you'll know which one as it's smaller and won't allow the selector shaft of the new trans to go in, some peoplehave had to do some clearancing on the inside of the case, mine fit without issue but if you do a dry fit and try rotating it if you feel drag pull it apart and grind that area......
seal up the 2 halves with RTV, as well as the endcap in the bellhousing area, your exhisting clutch and flywheel To bearing etc all remain unchanged but any 70-83 flywheel and clutch combo will work........
bolt the transmission back in, the trans wil lean to the passanger side a few degrees nto a big deal, you'll need to cut your trans crossmsmber into sections, reposition the sections weld them back together and brace up the seams, you'll need to have a new driveshft made using the slip yoke form the 240sx and the rest fo your old driveshaft to the length needed depending on your diff setup

it's not a bad as it sounds if you have some basic fab skills, if you can find an 82-83 280ZX Turbo 5 sped it's easier and a stronger trans, the Z31 guys are wrong about which trans is stronger

Phar

the vast majority of S30's came with 4 speeds, so a 5 speed makes the car a lot less annoying at interstate speeds as it adds overdrive, the 77-78 had the option of a 5 speed but they are rare and if you put the power to them they break, they just kinda added an overdrive to the stock trans and they did a poor job of it

Look for a rusted '82 or '83 ZX with a good drive train. The entire drive train and a few other bits will swap into a your 280Z. Unless you are building a turbo motor or an outrageous NA motor the Z tranny will hold up. This is the best L-series motor and tranny and they will directly swap into you 280 with virtually no modification. A nice "Bull Frog" cam (Do they still exist?) and a clean 2.5 inch exhaust and you will have an honest 210 HP.

Reggiegsd