as mentioned you will need to make some new engine mounts, if you use the T50 box and the standard KE25 gearbox crossmember it will line up with one of the gearbox mounting points under the car (there are two sets of gearbox cross member mounting points on the chassis rails, sometimes the second set is covered by underbody tar). GO a new diff diff and tailshaft. My car had a t-series diff in it whilst supercharged and it was ok, but as soon as it was turboed it snapped or twisted 4 axles. I would recommend that if you are fitting a diff without shortening it, then a t-series may be ok. If you go to the trouble of having something shortened then go for something stronger so you you only have the expense once.
You can fit a W series gearbox in with some tunnel mods. Mine has a W58 and none of the tunnel was cut up to fit it, just a bit of panel beating in certain spots was enough.