could be a lot of things... maybe even valve float but I wouldn't expect that till 5500 or so... I think most likely you're running out gas.... you get up in the revs and it just noses over... it won't necessarily miss, just stop pulling.

As for the carb, your 600cfm should be good for feeding a 305 well over 7000rpm. A bigger carb will not help you and might hurt low end throttle response. A bigger carb on the other hand will not necessarily make you run richer... this is a common misconception. Fuel mixture has virtually nothing to do with cfm of a carburetor.

What are the specs of your cam? It may be it just ain't meant to rev that high. Have you checked your valve lash? Springs? What intake do you run?

So if the cam is made to operate at higher revs (say up to 6000), the valvetrain is properly adjusted, your heads can flow enough to keep up, and your intake can keep up.... then it could be fuel delivery.... but those are a lot of "ifs"...

pretty helpful huh?

-Chris