I'm not surprised with your numbers. If you had used a performance intake such as a Victor or Trick Flow the 610 number probably would have been a touch higher. A 30 hp drop when bolting on the six pack intake seems about right. Did you do a weight comparison? You probably added 30 lbs when you lost the 30 hp.

My pump gas 496 with TF240 heads and a hyd roller cam that is fairly close to yours makes roughly the same numbers using a port matched Trick Flow intake and a Holley Sniper EFI unit. I would guess that a good intake would put you in the 620 hp range if the carb is correct. The six pack intake can probably be improved although not too many folks like grinding on cast iron. If it is a hot rod then switching to the aluminum six pack intake and having someone like Hughes Engines do a deep port match would narrow the gap.