dude, thats rediculous. you want to turbo your engine do a custom job.
or if you feel like paying like 5gs you could buy the engine...you can find a 2JZGTE and/or 3SGTE (both twin turbo) at nippon motors. they dont give a price, but their prices are like half of most people. engines are clean, ready to go. they come with a start guarantee and like 20k on them.
try
http://nippon-motors.com
and give them a call. im not too sure about toyota engines, im more of a honda man...but you can buy the engine for probably like 1500, then if you must pay for the swap, you can usually get it cheap if you know how to make friends...plus there are a lot of people who would do anything to work on a beauty like a supra...
good luck man. you got an amazing ride. make it go as fast as you can.