To my knowledge the VVTi ECU won't control the 4 speed, you would be far better off fitting the 5 speed that goes with it. (I have this problem in reverse, my 4x4 system will cost a small fortune to mount to the 5 speed, so I need to use the existing 4 speed and an aftermarked transmission control).
I'm not sure how well early LS400s hold their value in Hawaii, but here in the UK this conversion would be disproportionaly expensive for the value of the vehicle, unless it had some sentimental value!