The exterior dimensions of the 5.3 and the 6.0 are identical. The 4L80e doesn't taper as much as the 4L60e, so it will be taller under the cab.
As far as motor/transmission mounts, I would recommend something like
this. What you have to do as far as firewall/tunnel modifications will depend on where you position the engine.
You want to make sure that the output shaft of the transmission is angled down the same amount as the rear diff is angled up to minimize driveshaft vibration and maximize ujoint life.
