Torsion Bars... Best Diameter for a Lowered Street / Autocross car?

Just ask a lot of questions on Mopar suspension geometry. Just lowering the torsion bars puts all the other parts out of alignment. Factory mounting points were designed for a certain height and operating range. Heavy or lighter torsion bars are great but also designed to work under a certain amount of preload. If you don’t twist them enough or too much they do funky things. I’m no expert but it is why coil over parts are popular with MoPars doing what you seem to be working toward. IMHO.