You can fit a lsd into the standard altezza diff housing, and the std diff will hold the power i have been running 550hp through a std is200 diff since 2004, the is200 output shafts are 4 bolt and they have had no problems, the altezza shafts are 6 bolt so are inherintly stronger,
if you was to change the crown and pinion the altezza gearbox would read the same speed as the speed sensor is located in the gearbox, however the road speed would be out slightly, i can assure you there is no need to change the final gearing, your issue afterwards will be calibrating the road speed sensor, if its possible to fit the altezza one then that is the best option and it should be correct
my car was the first to have the 2jz gte conversion done in Europe
the mechanicals of this conversion are simple, in relation to the electronics