Carter Carb on a 273 HiPo?

One "maybe true" thing to check out is that I kind of recall that the early factory 4 bbl manifolds had a different bolt pattern (smaller) than either the later "standard" square bore pattern or the spread bore pattern. I think all after-market aluminum manifolds had the standard patterns. Also depends on year, which you don't tell us. An early manifold (64-66?) has a different bolt angle and size (smaller, more vertical) and are much rarer, as are the carbs.