Steering kit

I've been looking over the tech pages and instructions on the power steering upgrade and have a question concerning wheels. The instructions say....

Warning:
Using wheels wider than 7” with more than 3-1/2” of backspacing may cause the
right front tire to rub on the steering box. This will vary depending on the vehicle
ride height.

Was this determined by using stock drums or discs?
Thanks

Being that the steering box is on the outside of the frame like a 60-up GM truck the clearance and wheel selection recommendation needs to be followed. With drums the wheels are even closer then with disc brakes.