Hello GT,
The Ford part number for the kick-down rod in your application is D1ZZ-7A187-G. Ford lists the length as 19" and is identified with a green paint daub. This part was discontinued in '85, so it has been gone from Ford's parts system for quite some time. All my go-to NOS sources show zero stock. At this point, unless a forum member has one, I would contact Mike @ Motor City Mustang. He is a forum member/sponsor, has parts cars, and has helped many here.
If nothing shows at Mikes, try this source.
https://www.kickdownrods.com/shop/1971-1973-351c-c6-4v-mustang-cougar-transmission-kickdown-rod-
Be sure to read the notes about non-stock intakes and carb spacers. And in your application, Holly carburetors. They can make custom kick-down rods for these particular applications. This would be especially helpful if you feel any of your mods may be part of your fitment problem.
There is one other site I would suggest so you have some additional choices.
https://www.npdlink.com/product/rod-transmission-kickdown-control/205599
As with the other sites, there are contact numbers if you have additional questions.