As above, heavy duty is a very vague statement.
The red/white springs are a classic, but rare example in being softer and longer than standard 90 springs. They are only realy HD on a RRC with the self leveling unit fitted and working.
Some HD springs are made significantly shorter than standard to maintain the same ride height as before and others are both stiffer and longer than standard so will give a lot of lift in some applications if the vehicles weight isn't increased. There are litterally dozens of different HD springs to chose from, or if you like headachs then custom wound springs makes the choice infinate. to add another layer of complication, not all springs for LRs are linier, many are progressive, which makes comparing them and the resultant lift more difficult.
If you want a simple answer, just go to one of the reputable suppliers, tell them who much lift you want and roughly what the vehicle will weight and they will select the appropriate springs for your application. The ammount of accesories and cargo your vehicle carries will have a significant effect on the resulting lift.
I would despute the need to change dampers/mounts. To get full travel out of the suspension then these may well need to be changed, however just changing the springs will not render the vehicle incapable off road. My first RRC got a 2" lift with HD springs and never changed the mounts or fitted longer dampers. With some sensible driving it could still get most places at a pay and play. MY second RRC got off roaded for a while with lift springs and otherwise standard uspension before deciding on just which dampers to go for.
