Could be a 283 or a 348. They didn't start using the 348 Flags till '59. They sure would have looked cool on a '58. Not only just to tell us what was under the hood. BTW, if the Dual Exhaust are original, it's most likely a 348.
When I was young I never cared much for the 58's. I thought they deviated too much from the 55-57 formula. But, I have to say that over the years they have really grown on me. I'm to the point where I almost prefer the 58's because they're "different" Weird, I know.