OK, I just made some measurements on my cab.
The CL of the steering column opening is 12-5/8" left of center of the cowl.
The bend in the firewall is ~18-3/4 down from the top of the firewall (Measured at the location of the column opening)
The opening is semi-oval, with a ~2" wide circle at the top and a ~2-1/8" wide circle toward the bottom.
The Top of the opening is 1-3/8" above the firewall bend, and the bottom of the opening is 2-3/8" below the firewall bend.
That should get you close.
Also attached a couple of pics for you. Hope that helps
[on edit] just took look at your blog.
