A throttle body spacer on any Vortec is a waste. On the Vortec motors all that flows through the throttle body is air. So all you're doing is giving the air one more inch to flow through. The only benefit (aside from the whistle it gives the intake) is increased plenum volume. But it's so minimal you won't notice any gains. If you'd like to increase your plenum volume look into a plenum spacer. Summit Racing has them IIRC.
A TBI motor is a dfferent story. They work on TBI motors because both fuel and air flow through the throttle body. So the spacer gives the fuel more time to atomize.
On the Vortec motors there are some easy mods you can do to increase airflow. You can trim the "foot" off of the throttle blade. Delete the Vortec box off of the air intake. Also look into porting and polishing your throttle body. You can buy one that already has this done, CFM technology sells them, but they're expensive. Or you could look into doing it yourself. Spending money on these mods would be much more beneficial than a TB spacer. I'll do some digging and see if I can't find links to some of these mods. I know there's write ups on here.
Hope this helps. Good luck!
EDIT: There's info on both the Vortec box delete and the TB "foot" removal in Vortec Lover's build thread. Just scroll through all the pics and you'll see them
http://www.gmt400.com/forum/showthr...-Picture-Story-XD&highlight=vortec+box+delete