you got it right. I really like torsion bar suspensions- very clean. But you have to do it right. Besides that you are going to get a harmonic flex at some point that shaft is going to bend in these lengths. Drill it out. I am guessing but at about 3 feet, put the pressure on it and it's going to bow. Put a bearing in the middle and it.s going to shear-twist in two. If you are going to do this find somebody to do the numbers. I haven't been a design engineer in years. I think you can do it, If you don't run the numbers trial and error could be very expensive, as you know. I have a example,.keep it simple. Somebody I know built a piece of machinery with a 5 in shaft. the drive chain sprocket was mounted close to a bearing-he kept breaking the shaft. The set up should have acted like a torsion bar.the sprocket was so close to the bearing he had no torsion flex-He kept shearing a 5 inch shaft, 5in--A foot long 2 in shaft worked fine.I certainly know the book never replace expierence,B I have a math major,a physics major, and a mechanical engineering major, and taught 3 years in college, And I have been out too long to do this.I hope you do it, The other thing is safty-that thing can come out of there with terrible results. Do racing rules still restrainers, certainly scatter shields I considered a while before I posted this , being winded even for me, and pushy. I don't want to be bearer of bad news, so I apologize for my negative attitude,but I don't want to see anybody get hurt either. I may well be that I am out of the ball park, but till you do the numbers, you won't know.I will give my assistance any way I can.good luck. Mark A. Adams P.E.