There are websites online that sell "retro" kits. These kits can be used to convert an existing canopy but if your light hood is one of those plastic ones (thats what it looks like from the photo) then your probably gonna have to do something else because just about any higher output lighting setup you use is going to produce more heat than the standard bulbs and thus will proably end up melting the plastic hood.
For soft corals I would suggest a pair of 96W power compact florecents or maybe a pair of VHO
bulbs. What are the dimentions on the 58Gal Show?
You can get a prebuilt canopy that is made of wood or steel or you could build your own out of wood and use a retro kit to wire up and install you own lights. Depending on how comfortable you feel about it.