Most people don't know jack about the 1G. You should be an insider for us and find out EVERYTHING you can about it, and post it on this forum. You probably won't get many people who are interested in it since the 7M-GE has comparable horsepower and more torque than the 1G, AND there is no swapping involved; the 7M-GE comes in the cheapest MkIII Supras you can get your hands on.
I don't know if the 1JZ turbos will fit on the 1G, but if they are semi-custom and need a little welding/massaging, and you don't mind going with a little a custom work, you might consider getting a pair of US Spec 2JZ turbos instead. You can set them to work in parallel just like 1G/1J turbochargers. They are responsive enough for a stock 2JZ [280hp] and capable of flowing enough for the top end of BPU++ etc [450-500hp].
For questions like this, most people here don't know, BUT I would seriously consider checking out toymods forum online. I believe it's an Aussie site, but that's really what you want - they actually get JDM cars and some of those Aussies are creative as all hell when they find a project in the junkyard.