Torsion bars can loose their strength over time, that is why Chrysler made them adjustable, to make up for lost tension. They are just spring steel. I have seen a few break, but the circumstances surrounding those broken bars is unclear.

The average 426 Hemi was reported to be 150lbs heavier then an average big block (and a big block weighs 100lbs more then the average small block Mopar liked to use average numbers), but I suspect the aluminum heads and intake on your Hemi would pretty much cover that extra 150lbs (and if your "all aluminum" Hemi has an aluminum block, it may be lighter then an average small block). Unless you plan on doing road course racing or parking lot racing where you are really putting extra stress on the torsion bars, I believe your current torsion bars will be just fine. However, if you feel better dropping the money for new bars, go for it. What makes you feel better while your driving your car is all that counts.