Will heads off a 305 or 283 fit 350 Chevy?

No, heads from a 283 or 305 Chevy small block will not directly fit a 350 Chevy small block without modifications. While they're all small-block Chevys, the bolt patterns and sometimes the deck height are different enough to create significant issues. You'd need different head gaskets at minimum, and potentially other modifications to the heads or block to make them work, and it's generally not considered a worthwhile endeavor. It's much simpler and more effective to use heads specifically designed for the 350.