yes the r33 box is the strongest of the skyline boxes but are quite expecives, around $1500 mark i beleive just for the box
the r33 box was connected to the rb25 and rb 26 motors
the r32 box (which i have in mine) was connected to the rb20 motor and has the strength of a vl turbo 5speed with twin sincros (i believe).
is your nephew just wanting to keep the vl a crusier with no major power chasing or wanting the power? if so the power he is after then the r33 box would be the way to go and majority of the vlt 5speeds with decent power are normally using r33,
is there a specific reason why he said supra 5speed? if he has one laying around and was thinking of useing it i say dont get the skyline boxes and hold the supra for later as ill need it later on