Close (similar) only counts in baseball, horse shoes and hand grenades.

Those two shackles are High Boy only, not listed for anything else and I looked in both the 1964/72 & 1973/79 truck parts catalogs.

 

For those that go by numbers only and who want to put exactly what Ford had there - that may be the case.
As usual, that is very much un-needed & will probably cost more in this case as those exact shackles may be hard to find. So why not put something else that will function the same (maybe even better if you go to a different eye to eye length on the shackle) and be easier to find/cost less????

For those that go by the physical functionality of the part & what will really, actually work on the truck in the real world - "close" is by far fine in this case. As I said before a shackle is a very simple link device in the way it it used in a leaf spring application like on our trucks. If you've ever messed with customizing or even cycling a stock suspension just to see how it functions/works and what factors affect it, this would become VERY clear.

The front leaf mount, arc of springs and pin location in the springs are all far more critical than shackle length in this.

The rear shackle length can be changed as well as the mount itself to gain more droop travel & affect the way the springs arc through the travel.
This very well may not matter to the O.P., but I'm typing all of this to prove that a minimal change in his shackle will most likely (I'd give it a 99% chance) NOT be noticed by his as the drive.

Here's a couple pics of me cycling the rear suspension on my '74 F-100 supercab. NONE of the parts back there are Ford (guess what - they are Chevy... gasp!!!) and you can bet they'll function way better than what was back there stock. More than a few hours of messing with spring hangers, shackle length, etc. have made all of the above pretty clear in my mind

 

I gave a friend a set of nice shackels off a 2wd frame I cut up for his High Boy. He never mentioned any trouble using them on his truck.