I have been fuel injection for a few years but did the carb battles in a 304 with both Holley and Motocraft carb.

Personally, the Holley was a waste of money. I had constant problems with float and needle. I did have great success with changing the jets to where it was great in slow speed off-road performance but would not go more that 50 mph on highway. However, it appeared as fuel pressure was a culprit.

Overall, my best performance was when I added a fuel regulator to keep a constant 7 lbs pressure. However, that was not a consistent fix as the mechanical fuel pump can at times drop below its rated level and cause problems.

I finally went back to a Motocraft 2100 that was jetted for high altitude. Once adjusted, the engine ran smooth. However, I needed to adjust it frequently....

In short, check fuel pressure; it may be a fuel pump issue. Other than that, due to the quality of fuel, you may need to be running some type of cleaner solution to keep the needle moving free.