UPDATE: I've decided to go with an LY6 6.0L engine out of a 2011 Chevy 2500 HD 4x4 truck that was used by Fed Ex to deliver packages on rural routes. So it's just been cruising for mostly highway miles of which it had 300k. It's so much cheaper and I want to get this thing going down the road with a newer LS engine sooner rather than later. I'm going to save the LS3 or possibly LS7 crate motor purchase for a hot rod first gen Camaro or 67 Chevelle build. The LY6 I decided to buy was $350 for a core but with all the brackets pulleys, etc. It will cost me under $1,000 to rebuild it with new pistons, rods, totally rebuilt heads etc. etc.
For the truck also I have since changed my mind and will mate this 6.0L to a 4L80e automatic transmission. I have a bad left knee and just driving the truck around town with the Vortec 350 and SM 465 right now is starting to make it hurt, get stiff and it's just plain miserable. So automatic it is... If it weren't for that I kind of like shifting myself but such is life.