A 6 speed out of a 350z would crumble under the TQ of a 1uz, it can barely handle the power of a mild twin set-up on the VQ35.
I'm not sure that the BMW 850csi has the same Getrag as the MKIV Supra. As I remember it was the Version 5, or something like that. It can handle the HP, but is pretty different from a V160. The shift gate is a different pattern, and the throw is different. Plus by the time you bought a new V160 bellhousing (assuming they even line up in the rear section), you'd just be better off buying a V160, R154, or W58. FWIW, the W58 can handle around 500rwhp, maybe more if not drag raced from light to light. Plus people are letting those trannys go from the 2jz for pennies.
Hope it helps John,
Eric