Will a 4.3L V6 fit in Chevrolet Beretta?

No, a 4.3L V6 will not fit in a Chevrolet Beretta without significant and likely impractical modifications. The Beretta's engine bay is designed for much smaller engines, typically the 2.3L and 2.8L V6 options offered from the factory. A 4.3L V6 is considerably larger and wider, requiring extensive modifications to the firewall, chassis mounts, and likely other components. The cost and complexity of such a swap would far outweigh the benefits.