According to the 1957 Chevrolet truck shop manual, the 283 was only used on 2 ton and larger, the 265 was the option on 1 1/2 ton and smaller. There was also a 322 Loadmaster (Buick) V-8 that was used in 2 ton and larger.
I tink that '59 is right for the valve covers, but I have seen later heads that were drilled for both patterns.
