F$%k some people are negative.
We have heaps of people on here making ridiculously overpowered FWD cars... They can't get traction in the dry at 100kph but we still say "thats awesome" - just as an example of something that most people would find silly...
Then a couple of people come along wanting to do a relatively straightforward thing to a celica, and people just take the piss and say its pointless. I've had 4WD turbos for the past 3 years, and I've taken them to gravel in anger maybe twice in that entire time. The 4wd aspect is more and more becoming pointless to me - I don't drag it that often, and 99% of my driving is done on tarmac. The other 1% is grass hacking
I've got a VR4 at the moment (as per usual), and my spare centre diff arrived this morning. I'm getting it professionally welded up, and its going to take me about 4 hours to convert the car to permanent RWD. I can swap it back in half that time by just putting the axles back in. Since its my weekend toy, its MEANT to be fun to drive.
The GT4 I can imagine won't be much harder.
To spell it out, for any 4wd car - lock the centre diff up (done by opening up the gearbox, and welding it up like any other diff), remove the front axles, leave in the outer CV's to hold the hub together, blank up the holes in the gearbox, and there you go, RWD where you once had 4WD. Another way to do it is to remove the spider gears from the front diff (still with locked centre), however on some cars the speedo drive is off the front diff, so it may not be an option. Also means its harder to undo your handiwork.
I've seen it done to subarus, mitsis, hell - there is even a turbo RWD
primera up north done by this method. The exception is skylines with the ATTESSA system - you just pull the 4wd fuse on those. They are the only car I can think of with this ability.
Risks = Your transfer box and rear diff are under a lot more strain than in 4wd mode - they now have 100% of the power instead of the maximum 70% or so you'd recieve with a viscous centre diff. Your rear diff is in the same boat.
Gains = Car is ten times more fun. You now have skid ability, and you now feel dumb for saying the car was tail happy before. Plus, you don't have to swap the car you like now for a skyline/cefiro just to get a wheel going ow.
If you don't like it, put the axles back in. You're left with a locked centre diff (unless you have a spare like I have for this exact reason), but this isn't a huge downer - in mitsis particularly, its the only weak point in the gearbox. Helps with big launches too
Hope that actually helped.
). On further reflection I thought why would you take the front engine out anyway 
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at least im consistant in my hate