Just bought a 1984 Z28 with a small block400. It has a T5 with 3:73 posi. Do I need to upgrade to a T56 OR 6 SPEED?

 

Depends on what you want to do with it. There are a few options out there depending on how you drive. If you dont get after it too much and drive it somewhat conservative with only the occasional blast...then the T5 might be just fine. Just remember that there are members on here killing them with stock or mildly modified 305s so their torque ratting of 280 for a non world class and 300 for a world class can be taken with a grain of salt. The options you have are

T56 (this is a 6 speed)
Tremec TKO 500 or TKO 600
Upgraded T5 (drawing a blank on the company that does it right now)

Basically all have their advantages and disadvantages. You will have to do your research on this board using the search function to figure out what you really want