Have a look at this car here
http://www.trademe.co.nz/Trade-Me-Motor ... 255308.htm
It has a gen 3 3SGE with gearbox, axles etc... from an ST202 S3 Celica.
I built this car from scratch. This is how I did it.
The original car was a NZ new 100 series 2E powered Courier van (3 door hatch).
First I removed the motor, gearbox, axles, radiator, battery box and any ancilliory bits etc...
Then I hoisted the front of the car up high enough to slide the 3SGE/gearbox under the front. This had the gearbox mount attached but no other mounts.
I dropped the body back down to level, hoisted the engine/gearbox into place and bolted the standard Celica gearbox mount into the standard mounting bracket.
I believe that the gearbox mounting bracket on the chassis rail is the same for all models. But I'm not 100% sure. You'll need to check this.
I then supported the engine and bolted on the cam belt end mount. This has a flat bottom and when I lowered the engine it rested perfectly on the rail.
Using a piece of string tightened across the strut towers & a steel rule I could work out where the engine should sit and whether it needed to be raised or lowered at that end.
As it happens it needed to be raised just 10mm.
A small bracket was made & tacked in place. the motor was lowered back onto it and the holes for the captive nuts were marked out.
Once the bracket was complete it was welded back in place, painted and the motor relocated.
It now sat level & square but it wasn't yet secured front & back.
I then bolted in place the north south crossmember. Two things were apparent. First, the front mount was so far out (about 30mm laterally & 10mm vertically) a special plate was going to be needed. And 2nd, the rear mount slid perfectly into the mounting bracket but the hole didn't line up.
The front mount was dead simple. A piece of 10mm plate with 3 holes to bolt to the top of the crossmember and 2 more for the mount. Once fitted the engine/gearbox was locked in place and we could go to work on the rear mount.
This was also quite easy. I removed the standard mount and burnt out the rubber section. I then machined a nolethane insert the width of the bracket and pressed it into the mount. The mount was then installed and the position for the bolt hole was marked. It was off-centre as expected.
The hole was drilled, the steel collar pressed through and the mount reinstalled.
The whole thing now sat perfectly between the rails and between radiator & bulkhead. It was rock solid.
But as I discovered very late one night when attempting to fit the suspension etc ... in our excitement we'd forgotten to centre it.
The whole setup was 20mm off centre toward the passengers side.
We cured this with a custom made
(20mm longer) driveshaft on one side and a ST182 item
(20mm shorter) on the other.
I did look at the prospect of moving the engine/gearbox 20mm to the right. I was concerned about the closeness of the belts to the rail. It would have been very tight.
So the conversion is very do-able. My advice would be to use the ST202 gearbox. Internals are a little heavier than the earlier Celicas.
Flywheels are interchangable although the 3SGTE uses a bigger clutch.
There is a clutch plate which is the same diameter as the 3SGTE one and uses the same spline as the non turbo box. RomanV might be able to enlighten you there.
Good luck.