Having seen 1000+ harnesses, I can easily tell an aftermarket harness from an OEM at a glance. Connector styles, wire color shading, and very clean harness tape are easily identified as being reproduction. If you are looking for concours correct, you'd be better off finding a NOS or excellent used OEM version.
Otherwise, I would recommend AMP, as they are good quality and are used throughout the vintage Mustang world. I've heard reports of a couple of wrong wire colors for parking lamps, for example, IIRC. If I have to replace a harness and no used ones in stock, I don't hesitate to use AMP. My clientele, however, are rarely interested in absolutely concours correctness.
I always recommend the engine gauge feed and alternator harnesses to be reproduction, as quality used versions are almost impossible to find. I reject any of these harnesses that require splices that are visible, and most times they do require that.