In other words if its not a $$$$ concorse restored Mustang then why not just get rid of the rag joint?
All of my old Mustangs, and even an 80 Fox Mustang have been converted to a U-joint coupler, even my wife's 65 fastback.
Most of the couplers came with the rack/pinion steering kit, but I have since just converted from the rag joints myself.
I haven't seen the end of your steering shaft, but all of the shafts on my 65-67 I had to cut the ends off.
Buy a "D" coupler of your shaft size. After you get the coupler measure the shaft while its still attached to the steering box as if you had the coupler installed.
Cut the shaft about an inch up from where it goes into the steering box.
then grind one side flat for a few inches so that if you're looking at the cross section of the shaft it looks like the letter "D".
Next you buy a coupler with a "D" female, "D" female ends.
Take the part of the steering shaft with the spines, and grind it down on one side so that it too looks like a "D".
Since you have already ground down the side of the shaft that goes into the firewall a few inches up you should be able to test fit the coupler between the cut shafts.
Cut off the firewall side of the shaft to fit with the coupler attached to both sides of the shaft.
Once you have it where both shaft ends go into the coupler about an inch or so, use some thread lock glue, and tighten down the allen nuts to the shafts on the coupler.
You would be surprised how much better the steering feels after doing this.
Also, look for a kit, as there might be someone making it after all of this time.
Here are some couplers of which I speak.