I'd go with the 3rz setup personally.  Either way your gonna have to do some mods to it.  The 3rz would be a easier swap than the 5.3, since your gonna have to cut out your core support.  And the 3rz has more than enough power and torque for your rig.  I've known guys that dropped a 4.3 in and only to have their ass's spanked by a 3rz.  Ask Bigmike about him passing Dean on the way to Shaver up the four lane! 

The thing about the 5.3 is that it's putting out more power than the drivetrain can handle.  Many people have regretted putting v8's in their toys.  That's my