Sounds like you have enough info to go to your pitman arm manufacturer.
The tapered spline used in pitman arms is standard, or reasonably so.
If the folks who are going to make the pitman arm have ever made one before, or know what they are doing, they will be able to take your measurements and decide what standard size your have, and go from there.
If nothing else works have the manufacturer of the new steering box talk to the machine shop that will make the pitman arm, and take yourself out of the middle, the two machine shops should talk the same language.
When I needed a different pitman arm for my PS conversion I had good luck looking in the junk yard, as most every pitman arm I found would fit the gear box, I just needed to find the right shape for the rest of it, ended up with a chevy astro van pitman arm.
Grigg
