stock rear leaf springs from slant 6 better for drag racing

I just bought a 1970 Duster with slant 6 that I'm going to convert to 360 stroker. I came from drag racing 1971 Mazda RX2 with factory 4 link suspension and stock suspension. For stock chassis drag racing we always used really light springs in the rear to transfer weight to the rear tires. I am assume this is the case for leaf spring stock chassis cars as well? So wouldnt one be better off using stock slant 6 leaf springs that are lighter to transfer weight to rear wheels as well? I see a lot of people go out and buy fancy stiff rear leaf springs and dont know why?