Just to clarify a bit more because of your 5.0 Mustang comment.

In addition to what the others have noted, most people think of a "clip" as the front frame section, usually cut at or near the plane of the firewall, including the suspension crossmember and it's attendant componentry. The example that is in Spray's picture is really just a front crossmember (aftermarket fabbed in his case replicating the MII geometry), that is attached to his original frame front section. Due to the design of the Mustang II, the stock crossmember could also be detached from the front unibody section because all of the suspension attachment points are mounted to that crossmember. The 5.0 Mustang chassis (for refers to it as the Fox chassis) is a different matter. It is a McPherson strut design (typical of unibody designs) rather than a coil sprung, double A arm. As such it attaches differently. Though it has a front cross member, that only supports the lower part of the suspension, along with the steering components and frequently the engine. The upper attachment is on the inner fender panels (aprons) which would typically not be easily adaptable to your normal street/hot rod conversion.

And yes, front clips, and crossmember transfers (OE or aftermarket) are done on all manner of cars. Some are done well, some are an atrocity.