As for myself I have wanted to do a diesel swap practically since I bought the cruiser; a big part of why I wanted diesel was the reliability and the fuel cost advantages. I certainly wouldn't swap one purely to save on fuel, but the way I saw it was if I was going to do a swap anyway virtually anything would have more power, so I may as well get the best mpg I could get as well.
All that said now with diesel being higher, and often a good bit so, than gas around here that aspect is basically not much of a factor at all anymore. As for the reliability aspect I still think an all mechanical diesel is tough to beat, but I can get pretty damn close with the right gasser. I've simply put far too many miles on gas trucks now with very little mechanical issues to believe otherwise, and all the advantages of more power, better parts availability, etc are all still there.
I'm not arguing anything you guys have said; just up late with a sick little girl and bored so thinking out loud so to speak.
I've heard nothing but good about the LS swap in numerous trucks & cars - it seems to be a great gas engine. If I were going to do a gas swap it would be my choice, and I have never owned a Chevy.
My farm truck / tow rig is an '01 Dodge with a 24V Cummins. Daily driver is a FJ60 with a 12V Cummins. IMO the 5.9 Cummins is the best engine ever stuffed into a light duty truck. Is the 6BT more than you need in a FJ60? Yes. Is it more than you want? Depends on your expectations and desires. Mine isn't going anywhere until I'm too old to drive.