i have a stock ls t3-60 turbo 10 psi maxrev manifold tial 38mm wastegate. wat size downpipe should i get, i have 2 1/2 exhaust and planning on 3 inch soon, but wat size downpipe would be the best for my setup. also were can i get material for it.

 

The bigger the better. Go 3".

Try http://www.jcwhitney.com. Never bought from them but I hear good things.

 

were should i mount the o2 sensor, right after the turbo or like the stock location.

 

stock location, sensors dont like extreme heat, and you don't want to have to extend the wiring, it's a bitch