RPM airgap with carb spacer?

Okay, I thought I would go with a spreadbore. I will be running a mild 340, a rebuilt 904 and an 8 1/4 with 3.55s. Its gonna mostly be a street/track car as I am really close to Road Atlanta. I am gonna be aiming at 400hp, maybe a little more.
Best be getting a 8 3/4 if you plan on going to the track with an 8 1/4 and over 400hp.JMO