Not cheap, but I think a roller cam would be one of the best upgrades you could do. I think a good choice would be the Edelbrock Performer RPM roller or the GMPP LT-4 "HOT" cam. The Edel Performer RPM Air Gap intake is good for 10-15 hp over the GMPP unit. For the street I don't think I would go with a single plane intake. I think this would get you well over 400 hp. Your Vortec heads flow pretty good.
Taking a look at these may give you some ideas. If you don't want to spend the $$ for the roller cam ($800.00) you might want to try the flat tappet RPM cam.
http://www.edelbrock.com/automotive_...te_eng_cat.pdf