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Really if you're going to spend the buck to make a 850hp mk3 supra, you shouldn't.
The 1jz is just as good of a motor and while it may not be able to go as high as the 2j can in horsepower it makes up for it in throttle response/rpms.
The mk3 supra came with a 1jz in Japan. It's practically bolt in, all you need is a 89+ crossmember, 1jz wiring harness (although I recommend getting a USDM to 1jz harness, can be had for around $700), motor dress up parts, and exhaust. It's the cheaper little brother to the 2jz and it would require less extensive and custom fabrication to make it work.