Before you start buying a bunch of parts that you may not need, try this.
3/4" wooden dow rod or 3/4" diameter PVC plastic to simulate the steering shaft and good old duct tape. Make sure that the simulated "u-joints" are phased. This link should help you see what's going on.
http://www.progressiveautomotive.com/online009.htm
Use the duct tape to simulate the u-joints and the 3/4" for shafting.
Make sure that you have your headers/manifold bolted on, so you know what you're working around.
On Borgeson u-joints, Borgeson states that the u-joint can't run over 30 degrees from straight, on their single u-joints. Cut a template that is a triangle with 150 degree angle ( _/ , you get the idea). You can use this as a u-joint angle gauge. If the angle is sharper , then you need to add in another u-joint to relax it.
Ever Mustang based kit is different from one manufacturer to another. Some mount the rack & pinion straight to the cross member. This puts the output shaft at roughly a 45 degree angle from horizontal. We have "rack mounts" and roll the rack output shaft to roughly 20 degrees from horizontal.
My 48 with a SBC, Sanderson block hugger headers and 3rd Gen Camaro column, I have one u-joint off of the rack, one behind the header and a adapter at the column. Keep in mind that your motor could be in a different place than mine and the same with the column and front end.
When using only 2 "flexible" parts when the column if mounted at the floor, you don't need a support bearing. If you have more "flexible" parts than you need to add support bearings.
Mustang II Manual rack = 9/16" x 26 spline
1974-97 Ford power rack = 3/4" x 36 spline
GM columns = 1" DD or 1" x 48 spline.
ididit columns = 1" DD or 1" x 48 spline.
I do prefer the DD shafting. With this there is no way to put it together and get the u-joint out of phase.
P.S., we also sell Borgeson parts, but I'll only mention it one.