Oil pump replacement is usually the tip of the iceberg usually engine bearings or crankshaft may be damaged if the oil pressure is low. Verify the actual engine oil pressure using a mechanical oil pressure gauge. There is a 5.3 or a 6L engine in your Cadillac repair procedures differ. Autozone have some popular workshop manuals for free on line once you register (for free) on their website http://bit.ly/autozone_repair_info. There is also a company that independent auto repair shops buy their information from that the same information inexpensively to do it yourselfers. The information is year make and model specific repair procedures, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online
It's a big job and best left to a professional.