[Batmanntexas]

a lot depends on elevation where you are at. I am running the LS9 cam, 1-7/8 long tube headers, and full 3" exhaust. When I lived in Albuquerque, about 6000' feet above sea level, the stock 4" pulley was only good for 7 psi. I switched to the 3.75" and I came back up to 9 psi. Now that I live in North Carolina, about 350' above sea level, that same 3.75" pulley has me making a little over 11.5-12 psi. I am tuned very safe, and the car makes 650 RWHP on 93 octane.
If you want to remain below 10 psi, stick with the 4" pulley. You will lose about 1 psi of boost for every 2000' of elevation, and according to Whipple, gain about a 1 psi of boost per .125" reduction in pulley diameter.

-Scott