I've had discussions with some forum members who have found neutral safety switches (NSS) located at two different locations on '71 429 C6 Mustangs. I checked a microfiche catalog from 6/71, and only one NSS for a '71 Mustang C6 is listed, DØZZ-7A247-B. I checked my Mercury info, which shows the same part # for the '71 429 C6 Cougar. This switch is mounted at the manual shift arm location on the transmission.
These are mass-produced vehicles from the '70s, so variances (either by design or accident) and "Things" happen. Some devout Mopar and GM friends tell me their brands are just as bad as Ford's with these issues. I've dealt with these numbers for many years and still get shocked at some of the things I see.
I have also heard from some original 429 C6 owners who found the NSS at the shifter. This is the same neutral safety switch/harness as the FMX utilizes. Since your '73 started life as an H code 351, it was more than likely equipped with an FMX transmission, although on a smaller scale, some were equipped with a C6. If you don't see wiring where one may have been located on the transmission, pull the shifter cover/bezel and look for a tape tag with one of the following engineering ID #s, C8ZP-A, C9ZA-C, D, D1ZA-AA (Part # C9ZZ-7A247-C). D2ZZ-7A247-B (Motorcraft SW-1148) replaced the C9ZZ-C switch. This switch and others you see listed after 12/1/71 incorporate an extra circuit and plug for the hated seat belt light/buzzer system. The '71 shop manual does have one reference to the neutral safety switch adjustment at the shifter for Mustang and Cougar with FMX or a C6 with a 429 engine! Other than the one reference in the '71 shop manual on the '71 429/C6 NSS adjustment at the shifter, I've seen nothing else that would indicate that the NSS was located anywhere other than the transmission.
The '73-77 C6 equipped F series (regardless of the engine) has a steering column-mounted NSS. There were some significant changes in the '78/F series, and relocating the NSS to the transmission was one of them. So I would stay away from the truck switch and wire harness.
And just for the sake of confusion, my '71 M code C6 Mach 1s (Early build 9/21/70 and late build 7/28/71) have neutral safety switches on the transmission.
This is one of those times I would sure like to corner an engineer from this era of cars and ask, What.....were you thinking!?
The nut/retainer shown below is representative of the Ford 379114-S2 that is cataloged for the '71-3 lower fender attachment. The Ford version is discontinued, but it is something you should be able to find locally. The size is 5-16"-18, and the part number for the AMK pictured version is B13529.