An update to this, and probably a warning to anyone considering a Borgeson conversion.
First, power steer and manual steer cars use a different pitman arm. The Power steer has the bent pitman arm with a tapered hole to accept the ball stud (and this is why Borgeson sell the drag link adaptor for p/s cars)
The manual steer cars use the pitman arm with the ball joint on the end, and critically they are straighter than the p/s one.
The Borgeson kit does not come with a pitman arm, and you use the arm that was on the car originally.
In my case, I elected to loose all the p/s arms and use m/s steer stuff, because I wanted a clean look. I bought a new drag link, a roller idler from Open Tracker and a new pitman arm. Total investment about $500.
Now comes the pissoff factor - Borgeson have advised a Vintage Mustang Forum user that their boxes are 1/2" longer in the dimension of the sector shaft plane (so the box hangs lower). In their words, "it should not affect anything". Duh.
Then, the splines on my box are not fully cut, and you can see the difference in the 2 boxes; Borgeson nut is flush with the arm, OEM has thread showing.
So, where I calculate I have about an inch difference in the pivot point of the 2 inner tie rod ends, this is correct.
Still working through what to do. I could drop the idler an inch~ to level the drag link but not sure if that will exacerbate the bump steer. The interwebs have 2 different answers.
My advice to anyone considering a steering conversion with this stuff- don't. Rebuild the OEM stuff. This car steered very well before I changed over and if I had been able to sell this kit I would have used OEM. Perhaps using the original p/s draglink and pitman arm would reduce the bump steer to where it may not be noticeable, but a manual steer car is not worth it.