Alignment specs are in the eye of the beholder - what the arcs of motion of the suspension are, and how you wish to use them are things that can be "tuned". Those who have had an early Mustang or Cougar and performed the "Shelby drop" will tell you the changes can be dramatic.
Caster is the effect that makes your car "return to center" when you release the wheel - and that force has to be overcome when you turn the wheel yourself (gross simplification) - but it is a force to be reckoned with when manual steering is used in an older nose-heavy sedan.
1956 Ford Alignment specifications: Caster +1/2° to +1-1/2° preferred +1.0° / Camber 1/4° to +1-1/4° preferred +3/4° / Toe In 1/16 to 1/8 inch
Early Mustang Alignment specs: Caster +2° to +2-3/4° / Camber 0 to -1/2° / Toe In 1/8 inch
The Mustang came out with bias ply tires I believe - maybe try those for a start.....
Steve Metzger Tucson, Arizona