I can not expand the picture to see the detail.
The threaded rod end would be rather an easy reproduction.
Cut the old off and weld it to the new generic replacement cable.
These two vids may have some value
Adding rear disc brakes is a popular thing for older cars. So there is a lot of that going on in the market place.
The added length from OE is likely due to extended wheelbase to make the coach. The added length is 16" from what you can get to what you want. A 16" extension may not be that difficult to add depending on which end to extend.
What is wrong with the old one?
Do the replacements you have found have the same ends?