Rather than using 2x4 which can be a bear to rip and can be unstable, use 3/4 ply.
Rip it at 38 or 45 depending on your spring angle.
When you apply it use fine thread drywall screws, they are made for metal studs.
Apply it to the wall, into the ceiling shoot two nails in an x pattern that will hold the top of crown as long as it isn't too large (over 5 1/4) and you using MDF 4 1/2 if it's pine... I suggest MDF as it's more economical and stable, takes a better finish.
For more pop by the tutorial on the altereagle site.