Yep, 3/4" x 36 spline at the box. Double-D at some shafts, or 3/4" x 36 up at the column.
If your local yards have older Ford trucks (and even some GM ones) you might be able to find something, or piece something together that can be made to work.
Just make sure of a few things:
1. That it's not also worn out. The shafts are usually good, and the rag joints can be replaced as can the original u-joints. But by the time you buy any new replacements you're into if for the same price as going all new anyway. Well, almost anyway.
2. I would not get one that has the rag-joint at the box. They're less forgiving of odd angles between the box and column, and tend to be quicker to contaminate the rag joint material over time when exposed to power steering fluid and other stuff.
3. And last, make sure that if you do find one with a rag joint and collapsible shaft that has the right u-joint splines and column interface, that it's not the reverse of the Ford joints. There are those with the rag at the column end, and those with the rag on the shaft side.
Either would work, but has to be mated with the correct interface of you'll never get them attached.
There may be shafts that are close in other words. You just have to be lucky enough to find them.
Paul