You will want a bigger carb when you get that better exhaust, like a 750.

Also double check your valve lash and make sure its not too loose. Are you valves floating at 5500? Did a professional set up you head springs or are they stock edelbrock?

About the brake drag thing, if you can jack up the car and spin the tire freely without drag noises its fine. If it only turns a couple revs and stops, I'd have it re-adjusted. My 408w picked up 30rwhp at the dyno after breaking in the rear drums on my drag car, the stang's emergency cables were too short for the LTD arms/drums and made horrible drag/grinding sounds and didn't spin too freely at fist install.