What is great about Gen I small blocks is also what can make them a pain to ID. I built a 350 for a '63 C10 one time and it sported a 327 intake and 283 valve covers. So just because the intake says '66 283 casting numbers, don't trust it to ID the block. Even 283 or 327 heads could be sitting on a 350 block. That's why the only definite answer will be to get the casting numbers off the back of the block. Besides, it may be worth pulling the motor just to inspect that bottem end since you said in the other post that you will be going through the top end.
