Is this the steel that you're using Gargo, or did you just cite it as an example?
That is just an example. Truth be told, I searched for 4043 and didn't realise that the search engine had included results for 4340. Good call Ron as you should not be using this steel for ARBs, but be using a spring steel. I am using a modified Peugeot ARB.
Beryl already has a 1in Hex 4043
The only 4043 I can find is an aluminium alloy, 5% silicon. Sdibbers is what you are using? Or have you too made a typo?
For the Hex, you'll need to use a different equation for J the 'Polar moment of inertia' but you'll find it is very close the round bar equation.
Either way the spring steels I could find info for have a Shear Modulus of 10x10^6. Which is the base for the calculated number that appears in the spreadsheet. (My example used 11.5x10^6)
The other simple thing to do is measure what the ARB you have. Clamp one end, support the other and torque the bar, measure the deflection. Convert the result to the correct units.
In the real world cars are put on a rig that measures all these values to validate the designers numbers. The rig pulls and pushes, deflection and forces are measured etc.
In the image below, the rig is a post on each wheel, with the 4 golden jacks deflecting the chassis for roll and pitch. The weight transfer in roll/pitch can be measured. Sorry for the horrible car, but could find one with a Rover. These simple rigs can also measure the true CoGs, but overall and front and back CoG. Which I have done with my own car using bathroom scales, raising the front of the car and measuring the weight shift to rear. Simple but effective.