3746829 283 57-61 Cast Iron, 4-bbl
3746829 283 58-61 230HP, Corvette, Cast Iron

Not to take away from anyone's fun, but to help you learn.........
I got this info by typing your casting number only into the Google search bar. This was the first hit.

If your manifold is original to the heads then it's a '57 or 8. And to fine tune that, go back to my motor mount boss comment.