Torsion bars are a different type of spring.
Because torsion bars ARE the pivot point, they have a motion ratio of 1 to 1 (1:1).

The length of the lower control arm is irrelevant to the motion ratio of a torsion bar.


However, the length of the lower control arm IS relevant to the pounds of force that the torsion bar applies at the wheel.
This amount of force at the wheel is called the wheel rate.

Imagine for a minute that the lower control arm is a lever that you are using to twist the torsion bar.
A longer lever makes it easier to twist the bar.
This means that a longer lower control arm will have a lesser wheel rate than a shorter lower control arm.

Here is a website with an explanation and formulas for calculating the wheel rate for any torsion bar.
It also has a calculator to do the math for you. smile
You'll need to know the length of your lower control arm,
the active length of your torsion bar,
and the diameter of your torsion bar.

https://swayaway.com/tech-room/torsion-bar-wheel-rate-calculator/

1962 to 1972 B bodies & 1970 to 1974 E bodies all use torsion bars with an overall length of 41 inches.
The hexes on each end of the torsion bar do not twist. Only the distance between the hexes twists. That distance between the hexes is the active length, and it's 39 inches.

In most cases, the active length of your bar and the length of your lower control arm will never change.
This calculator is primarily used for comparing the wheel rates of different diameter torsion bars.
It can also be used to compare the wheel rates advertised by different bar manufacturers.