If you do go for a 6.0, check the shape of the transmission pan before you buy it. Some came with the 4L80e and some came with the 4L65e. Most of the 4L65e setups were in the Cadillacs, but not all.
You can swap transmissions, but it requires both a segment swap in the ECU and some wiring changes. The guy that runs LT1Swap has a $75 ECU programming service that I have never heard any complaints about.
