Looks like a mixture of holleys. A modified sounds better. You'll have to remove the carb from the intake to find out what size throttle blades/bores are on it so you can determine what cfm it is.
It's sometimes difficult to determine what exactly it is once the choke housing has been lopped off. Based on what I see, I'd venture to guess it's a 600.
Most of the vacume secondary holleys didn't have a secondary metering block so its kinda hard to say whether its original or not.
http://www.holley.com/data/TechServi...%20Listing.pdf
There's some info there. I also recomend getting a book that details how to rebuild and modify holleys.
From what you say though I'd be lookin at the primary accelerator pump and the power valve making sure everything is setup to work together.
Don't give up to quickly, even a brand new carb out of the box will require some tuning to the power valve and jets.
As said above, vacume guage required.