[mattfranklin]

You say you've tried less timing.
Have you experimented with 5 deg. more?
It's something very easy to try.
When I have had crappy running engines a little more timing sometimes helps smooth things out and gives you a chance to work on the other things.

Also, was your 600cfm carb a known good carb?

BTW, I agree on the accelerator pump cam.
I think the Holley "yellow" one is the most aggressive.
I have an Excel spreadsheet from Holley that lists the curves for all of them.