I have the Holley Sniper on my 408 and it works great. Hit the key and go. I had a few problems in the beginning but those were due me not setting it up correctly. I don't control the timing or anything, let it "learn" and it's been good to go.
I did run that same carb as you on a 340 and it was flawless, not sure what problem you're having, mine did work great, but the efi is much better overall.
