Also, why the difference in king pin inclination ??
Is one more suited to modern roads or such ??
Ricky.
I don't know the king pin inclination of the two axles but, I can tell you why it is used.
king pin inclination does two things, first it keeps the car or truck wheels pointed forward and does that because when the spindle turns forward , it pushes the tire, into the ground, creating a pressure on it, it does the same if, it is turned to the rear. There is no added weight, when centered, so, it whats to go straight. You can also make a gate, self closing and self centering by using the same type of inclination but, in the opposite direction.
The other thing it does, is center the steering axis on the contact patch of the tire (actually a little to the outside). That reduces the steering effort, as much as possible. If you've ever put after market wheels on a car, that have a positive offset and with no PS, you will notice a increased effort to turn the wheel, when setting still. That is because the tire is now moving in a arc, rather than turning on center. The Mustang in my avatar, had 8.5 wide front wheels with a 1.25 positive offset. If I hit a bump or rut, on one side or the other, it would pull to that side.
That said, if the angle is increased that much, maybe Ford found that the Twin I Beam was harder to stabilize. I do know, that when I bought my new '64 Ford van, it tended to wander. The fix was replacing the radius rod bushings with ones that had less give but, still with more inclination, it shouldn't have need that.
That arc, also creates leverage on the tire and will also cause the car to want to dart, in the direction of bumps and ruts.