Good morning, I'll chime in on the reliability aspect of wanting the 1FZ. A little research will yield all kinds of information about the reliability of the Vortec 5.3 L and 6.0 L. These motors are iron block for a reason, these are fleet motors in use all over this and other countries. The Vortecs mentioned see 300-500K mile on just oil changes and spark plugs. This meets or exceeds the numbers being posted by the 1FZ.
Parts availability is a non starter, you can build the entire Vortec motor out of an Auto Zone. The HP and torque numbers speak for themselves. The big winner is pure drive ability. If you want an idea of how it feels, go take a used Tahoe or Suburban with the Vortec drive train for a ride and imagine you are driving your cruiser.
What is really cool, with the Vortec you can choose to slow down and see all that scenery everyone talks about as a proponent of the stock 6.
In the end it really is not one single reason a swap makes sense for a lot of people, its all the positive changes that add up. I look at it as an investment in the future of the truck. Like any other mod you do to your cruiser, this one is not for everyone. But if you plan on keeping your cruiser you will have to include rebuild or repower in your long term plan.