'79 auto has T series 3.727 (41/11).
'79 manual has T series 3.583 (43/12).
Any T series can replace it but if the ratio is wrong then your speedometer will be wrong.
It will also affect the top speed / acceleration trade off (one increases, the other decreases).
Look here to see what else has your diff.
http://members.iinet.net.au/~stepho/celprod.htm#cel42pr
http://members.iinet.net.au/~stepho/diff.htm#sets
Beware that if you get a diff from a model with IRS (eg '83-85 Celica GT-S or Supra) then you need to get a shop to pull the old and new diffs apart and make one good one from both (old carrier, new spinning bits).
Also beware that some models like the 82-85 Celica had both T and F series diffs as options and they can not be interchanged into the same axle housing.
If you are adventurous, you can put a 79-81 or 82-85 Supra rear end in as long as you do the whole rear end (ie entire housing, all suspension, etc).
But converting to IRS means cutting off the panhard rod mount (the point of no return) and drilling VERY accurately new holes for the IRS K frame.
In my opinion, disc brake rear ends are overrated unless you are into racing.
If you convert to disc, also use the donor car's rear bias valve.
- Stepho