Hey dude,
firstly :woot: to another aussie

:
anyway, to your question....3s-fe to 3s-gte is not possible. 3s-gte in place of 3s-fe is possible. what i think you are trying to do is a 3s-fte conversion which a few others on this board have done. essentially what you will need to do is get the manifold from the gte and turbo if you wish. if you do get the ct-26 then you will need to connect the associated oil and coolant lines to the 3s-fe.
Computer wise you wont be able to use the 3s-gte if i'm right (some of the other guys will verify this) what would be your best option is to get a piggy back or standalone. what some of the other guys have done is use a vortech FMU, either way you will need to do some good tuning else you will have a grenaded piston ( 88LE can tell you more

: )
You can use the 3sfe head but not the 3sgte head, they're different. transmission wise you can probably use your current one ( which is? ) but it also depends on the amount of boost that you decide to run. if you do decide on running higher amounts of boost you will need to do a good rebuild on the 3sfe and may need to find a stronger gear box. One of the guys in sydney is running the 5spd 3sfe ( s51 i think) gearbox, only he had it built up somewhat, i'm not sure on details. you can also use the mr2 (e153) gearbox, or the 4wd celica box but you will need to modify some aspects. then you can use the celica ( s53 ? ) gearbox.
hopefully that helps you enough... try search 3sfte, turbo 3sfe etc should yield a fair bit of info...
Cheers
