Most header installations, regardless of the brand, require lifting the engine and shifting it to one side to mount the opposite header. Some vehicle applications allow for easier header installation, but a vehicle with factory shock towers will always present challenges. Header installations may include the removal or disconnection of other components. This is common for many vehicle applications.

To clarify. If the contact with the mounts ocurred during the installation proces, read the comments in the above paragraph. If contact with the mounts occurs after bolting the headers to the engine, this is a separate issue. If someone has replaced the engine mounts over the last 50+ years, it is possible they are not OEM. All Sanderson Headers are designed for vehicles using factory engine and frame mounts unless otherwise specified. The FF393 header set will not contact OE mounts on an original Mustang with the FE engine.

Starter clearance is a common issue when installing headers, particularly in certain vehicle or engine applications. Ford FE engines are prone to starter clearance problems, which is why we recommend using an aftermarket starter, such as the Powermaster 9500-series. We explain starter issues in this article:
Understanding Starter Clearance Issues