Jani,

Couple things I can add based on picture and comments.
Block would be '58 or later based on motor mount flanges.
I don't think the color is meaningful for a 283 or 327, the early blue Vettes were the sixes, by '58 they were the signature "orange". In the '70's the "corporate" 350's were also blue.
Based on the casting mark, heads should have a casting number ending in 520 or 896. They'd be for '58 & up 283 power paks, '58-60 Vette, originally were 9.25-1 comp ratio, 1.72/1.50 valves, 59cc chambers.
Wouldn't be from a '66 Vette unless it's a 327. With the introduction of the 327 in '62, 283's weren't available in Vettes.
Those valve covers weren't available on the 327, but then, we don't know how stock it is either.