Thanks Mike, that is what the first 2 casting number sites told me as well, '57 Corvette up to 290 HP, and I was pumped.
However, several other sites list it as both 265 and 283, which may be true because when the 283 came out in '57 it was a bored 265. It wasn't until late '58 that GM cast a 283 specific block. That is why you can't punch a 265 or a '57 283 out to 301 cubes. It has to be a late '58 or newer 283 with side motor mounts to be able to take the 4 inch bore.
