Bill, I drilled and tapped one from a 5/16 to 3/8 once and that fixed the problem perfectly. I'm not familiar with the Holley carbs, so that may not be an option. 1/2" seems large for a fuel inlet compared to what I usually work with and I don't know if 9/16" is an option.

Jeff

My 1953 Chevrolet
1947.1 Gallery
1972 C-10 1/2 Ton & 1972 C-30 1 Ton