What do mean putting a supra in? Like, the engine and all or just the 5 speed gear box? If you want to put the engine and all in you'd definitely need to get it engineered and such. Personally I think that sort of conversion is sacrilege anyways.
But if you just mean the box I'd be interested in finding out too because i want to put a t5 box in my vk. Although the vk's did come with a 5 speed box so maybe they wouldn't notice it was a different breed of box when they did the roadworthy . Depends on who you get to do it, one mob might pass it just fine and another might pick you up on it and demand a mod plate.
The gearbox conversion is legal and has been done to older cars many times.
Dellow and CRS sell kits for this sort of stuff and was going to do it myself to and EH project I once had. I would buy a kit (atleast the bellhousing and the gearbox crossmember as it will need to be a modded one) and go from there.
Mal Wood Auto in QLD do conversion kits as well. I have a supra box in my VH, and got most of my parts shipped to me from them. good quality, fast delivery and at the time they were much cheaper. They can also make custom 2 piece tail shafts to suit. U gotta be careful - i didn't realise until id put it in, but second hand the single piece shaft i put in my car vibrates a lot, particularly at high speed. I'm holding out at the moment, but be putting a new 2 piece shaft in when i change from a Salbury to a Borg Warner diff in the next month or so.
PS not sure of the legality's involved with fitting the supra box, but it makes a hell of an improvement over the traumatic i used to have