Two different versions I know of were used for this wire. My understanding is it is an early/late '67 thing (all 68 same) but have yet to determine a cut-off date. Early version runs alongside the headlight harness, over the brake booster along the pinch weld of the cowl and to the connection near the right hand shock tower. The late version was shortened to run along (clipped to) the right hand shock tower support (to the same connection point).
Being you are working on a Shelby, I would consult one of the Shelby Guru's and see what they can offer.