Save money for it!! I would like to put a 5.3 in my Jimmy. I like the power of the 4.3 but the 5.3 would be awsome. Same great milage or better with minor programming. At least for 96 and later, the swap is easier. The original PCM is kept in place for basic functions like the instrument cluster. The 5.3 PCM is added for only engine functions. A custom harness makes the swap easier. Here is a conversion:
http://www.hotrodlane.cc/PDFFILES/Lisa%20Tahoe.pdf