Since we are talking about timing and carburetor problems, we probably should move this posting, but I had exactly the same problem with the Holly 4V on my GT. I found the problem AFTER (!) I changed the timing gears and chain, and it was the carburetor all the time. There happened to be a speck of debris/dirt/rust (who knows) caught between the needle and seat for the float, and the needle couldn't totally stop the flow of the gas from the fuel pump.
I really cleaned the needle and seat and it cured the problem. I will say that the original camshaft timing gears used on the 289/302's were plastic, and they do wear out and jump time, but before you tear into the front of the engine, just check out the needle and seat of your carb.
Hope this helps,
Russ