Frank, first welcome to Lextreme. Second, which Japanese V8 twin turbo are you referring to? AFAIK, neither Toyota nor Lexus are building any UZ motors with twin turbos for their street cars?
Since you have a late model GS and you're in the US, you need to understand that extensive mods like this will affect the OBDII system, therefore anything you're thinking of doing will have to be for racing or offroad purposes only, right? You're not going to be able to pass an OBDII scan after mods like this, and for sure you won't be passing any stringent emissions testing.
Right... now that that's out of the way.... I honestly wouldn't attempt something like this if you're trying to do it on a tight budget. There are no bolt on kits yet for this, and if there were, they'd still probably cost in excess of $5-6K even with a sweetheart hookup. Then if you don't do your own labor, you're probably looking at another $3-5K or so to put it in. I don't know you or your background, so please don't take offense, but it's a lot more than just slapping a couple of turbos on the motor. In parts alone there's the turbos, the new exhaust manifolds, the intercooler(s), the BOV's the WG's, the boost controller, the piping, etc, etc.
Plus you're going to need a new set of injectors, a piggyback fuel controller, probably a wideband AFR system, a new set of fuel pump(s), and most likely a new exhaust system..... And yes, probably a high stall torque converter and a beefed up tranny. The A340E's from the older cars don't hold that much power, and although you probably have the newer A650E in your car, I doubt if it's that much stronger.
Honestly, if you just want some cheap power, for some occasional blasts out on the streets and freeways, consider putting nitrous on the car, with the associated mods needed for that. You can still pass OBDII and you can leave most everything else stock, other than needing some tweaks to the fuel system. I think you're in for a real money pit of a project, otherwise.....
