Will a auto trans from a 1st gen IS work with a 2nd gen GS ?

 

Yes but you will need a custom drive shaft made, the output shaft is different.

EDIT: Here is the thread where I asked the same question-
https://www.clublexus.com/forums/per...rivetrain.html

 

Damn!!! Thanks I'm not really planning on doing the driveshaft route .I wanted to know because there was a seller on my IS who was selling his trans that had the valve body upgrade and sprags done for $1,250 and I wanted to buy it . Hell I might as well just get my trans beefed up instead.

 

Yeah I was debating the exact same thing a few months back. I don't want to settle on vb/sprags alone, so I want a full build. I have a used GS tranny with the vb upgrade sitting in the garage, so I'll probably have that built in the future.

The SRT vb/sprag upgrade is like $800 plus shipping both ways, so I think you are better off getting yours built.