instead of questioning the guy as to why he would want to do whatever, let him do what he wants, it's his car. ;)
anyways, OP, better and cheaper method would be to use 2LS 4piston calipers and rotors, they are MUCH MUCH cheaper than the Supra TT brakes, and are actually lighter and offer ~85-90% of their braking capability for ~40-50% of the price.
to put 95-00 LS400 front brakes into an 89-94 LS400, you need to replace the calipers and rotors (both will bolt right in, no modifications needed), and 95-00 front LOWER BALL JOINTS because the calipers will not clear the original 1LS ones as they are not notched for clearance. also, you will need wheels to clear the brakes, High disk wheels for sure, just get some that will fit a big brake kit and you should be fine.