Depends on what kind of power you want to put through it. If it is just an 18R/RG, then I would just look at getting an LSD for your T series diff and putting some RT141 corona discs on it. If you are planning on more power then an upgrade is wise.
The early celica diffs are very narrow, so I doubt you will get one to fit your needs easily. I am going to be using the MX23 that Norbie recommended with RT corona brakes. There is heaps of options for LSD's for these.