i think i would go for the rebuild. it should be easier than a swap, and the 6.0 is an iron block as opposed to the aluminum blocks the ls1/2 have. wouldn't make a huge difference, but it's been my impression that the iron block will be better suited for the higher heat that is produced from doing "truck" things like towing heavy loads and wheelin offroad. not to mention you could rebuild it with some high end forged internals so it can handle all the power you can put on it!
just my two cents.
2000 ECSB 5.3L with K&N CAI, true duals with flowmaster 40s, and 200k miles!