Adjusted the title to reflect 1967 eight barrel's focus so that we can look at practices at that plant and time period
I was looking through my Electrical assembly guide and there is no shown clips for the 63 body. What is correct as far as sending wire retention where it lays across the tank in the trunk?
For your car, plant, body style and time period there was only one clip/retainer for that wire used. That clip was one placed over the drivers side wheel house pinch weld (over the normally black luggage protector strip) From there over across the trunk the wire was held to the body with a series of short sections of friction/gaffers tape - from my observations and pictures three to four sections were used as illustrated in the pictures of original cars below. Hope this helps
Early style retaining plastic clip - later cars also used the sheet metal style
First tape section on cars closest to your car can be found up here -
Cars a little after your cars date can be found here also
A couple of additional tape stripes to hold wire up and out of the way so that the wire could be installed prior to the installation of the gas tank
For extra credit
after the wire exists the trunk it makes a hard turn and is held to the gas tank opening lip with two sheet metal clips/retainers
......There are photos of the original clip (381645-S100) in a thread over on the SAAC forum (to confirm that Shelby is not different than a Mustang). If a person needs "original" clips and maybe is missing them, pictured also within the SAAC thread are valve cover wiring J-clips next to the correct clip since they can be substituted in a pinch with a little tweeking.
That tread focuses on 69 Dearborn cars