Does a 4.6 engine fit into V6 mustang?

A 4.6L engine is a V8, not a V6. Therefore, it will not fit into a Mustang designed for a V6 engine without significant modifications. The engine bay is simply not designed to accommodate the larger V8. You'd need custom mounts, potentially modified or replaced firewall, and likely adjustments to many other components. It's a major undertaking, far beyond a simple swap.