the torsion bars are available in a # of thicknesses from mild to stupid stiff
the most common are the .88 small block , the .92 R/T bars & the .96 upgrade bars , also the 1.00 bars , you can definatly upgrade without being overly stiff , I know 1 guy that road races & his car will bottom out at over 140 mph with .100 bars
but for most street use .92 for small block or .96 for big block definatly helps the handling
the bars will swap between 64-72 B bodys & any E bodys but new ones are cheap enough so I would just get new ones
there are numerous topics on brake swaps already , basically 71/72 B bodys , 73-75 A bodys & the controversial F body spindles & brakes will all swap