This is a very bad idea, since the factory 5 speed is geared for the 200 Horsepower naturally apsirated Supra. To install the six speed requires changing the entire driveline, and when complete, the car will respond slower than with the factory 5 speed or automatic. However, there is a 6-speed transmission for the NA MKIV. Although they weren't offered on the american version, so it would probably be quite hard to acquire one. Even if you were able to obtain one, there might be fitment issues.
The 2JZ-GTE head will not fit the 2JZ-GE
The 2JZ-GTE head will not fit the 2JZ-GE