SS or XHD springs for 66 Dart?HELP!

Here,s my dilema.When I got my 66 Dart(6cyl.,13 inch tires)the leafs were saggy.I installed a 8 3/4 with 15 inch rallys,tires are 245/60/15 TA,s.I installed a set of springs that I picked up from another 66 Dart and they also seemed to sag.So I installed air-shocks til later.Well later is here now and I,m wondering which springs to go with?(Car is for street/strip)also now has 360/904 and disc brakes up front.Which springs can I put on her?
SS..?
XHD..?
or just get a spare pair re-arched?

Thanks for any suggestions or part #,s.