SUPER STOCK or XHD LEAF SPRINGS?

I would like to raise the rear of my 73 Hemi Duster to run 15x8 "steelies"
with BFG 275-60x15 Drag Radials.Currently the car has stock 340 springs with an extra leaf.
Mancini offers the Super Stock Mopar springs as well as a spring he refers to as XHD (Extra-Heavy Duty).
How good (or bad) are SS springs on the street? Are they overly stiff?
How much will they raise the rear?
Anyone know anything about the XHD springs?