I've been putting Vortecs and LS3s in 60 series trucks for a while now and although I would not call myself an expert on the LS motor platform I probably have more experience than many on this forum. My OPINION, its a game changer. If you are being honest with yourself you can admit that these truck (gas at least) were under powered when new. Putting in an LS motor ends the days of keeping up in normal traffic being a pain, merging on to the Interstate at 45MPH and being a moving pic, passing at 60MPH plus without planning for two miles and doing it on a down hill run with a tail wind.
It is not all about big HP numbers and torque. It's about rolling into the throttle coming from a light rather than tromping on it just to get out of the way. It's about rock solid reliability. Folks go on and on about their stock motors having high 200 to 300K miles on them. LS motors are just warming up at those numbers. With regular oil changes Vortec motors in fleet service clock 300K-500K with regularity.
My favorite are the guys that state "I like the OEM motors being slow, I like taking my time driving through the country". The owner of an LS swapped 60 or 80 have the OPTION of slowing down any time they choose to. Bottom line on all of it, at least for me is I see no advantage of rebuilding 30 plus year old technology when modern power has been well documented as a viable alternative.
Once again, this is my OPINION. I've not pulled out a diesel and installed an LS in it's place yet. I have looked at a diesel 70 series for sale with the thought of selling off that diesel to help finance an LS3 swap, for myself. But that's me, my thought is, a diesel is perfect for my tractor and a pulling truck if the need arises. I do not want one in my SUV. Precisely why there is chocolate and vanilla.