You may want to check you're exhaust tips. If you can see allot of black carbon, it may be to much carb for it. Mine had that and it also had a tendency to back fire from time to time when. I had a 600 on it then, switched over to a 500 Edelbrock and all the problems were gone. No back fire and no more black carbon.

 

600 cfm is on the large side for a 289/302 but is certainly workable. You could go through the calibration process.
http://www.edelbrock.com/automotive/eps_sect1.html

Your "chug" at idle could be an aftermarket cam, but it could also be an indication of the need for a tune up or a vacuum leak. I would start with the basics before reworking the carb.