what size torsions for a 69 dart for a little better handling?

building a 69 dart for a friend, its a /6 car that will be getting a 6.1 third gem hemi. keeping stock style suspension but wanting it to handle better without beating his brains out. we will be doing f/r sway bars and the full us cartool frame stiffening/minitub/spring relocate, ot sure what diameter torsions to get, what do you guys reccomend? thanks guys!