I had the same problem when I upgraded to power steering my truck would dart all over the road. I took it in for an alignment at a local shop that has years of experience with hot rods. I was told they like to see 4 to 5 degrees of caster with power steering. With the Mustang II suspensions where the upper mount is welded to the side of the frame the control arm will hit before you get that much, not all Mustang II kits are designed that way. I found 2 options for fixing this problem:
Option 1 - I ordered 5/8" narrow upper control arms from JW Rod Garage. They don't list them on their website, I called and talked with Daryl the owner who was really helpful. I now have 4 degrees of caster and the truck drivers great.
http://www.jwrodgarage.com/
Option 2 - Adjustable upper arms, made by SPC performance. I think these would work well also
https://www.summitracing.com/parts/sps-94800/overview/