The 265 were used in 1956. 1957 did offer the 283, but not many people realized it because it was never advertized.
The offset valve cover holes were changed in 1959. They may have been changed over in late '58, but I'm sure it was the 1959 model year when the holes were changed to inline style.
Many parts guys sell offsets and list them for 1956-57. My 1958 Corvette and 1958 truck both have offset valve covers and they are number correct vehicles.
