I'm a little late to the motor discussion party on this one. First off, I have always loved your crew cab. It, along with other ones I saw on the internet, coaxed me to want to build one into a tow rig for myself instead of buying a new 3/4 or 1 ton tow truck. There is something about these crew cab square bodies that just screams awesomeness 
So, don't know if you have seen my build to my crew cab yet or not (link in my sig), but I went through all the same processes of you when deciding what drivetrain I wanted to swap into mine. I looked at 12v's, LS's, 4l80e's, Allison's, pretty much everything. I decided on an 8.1/NV4500 in place of my 350TBI/TH400. Obviously my situation may be different than yours and I may want something different out of my truck than what you really want, but I found that swapping an 8.1/NV4500 into these crew cab squares is about as straight forward as you can get (even easier than a 6.0 since you don't have to worry about conversion motor mounts and exhaust is a little easier to deal with). Also, it was going to be cheaper/easier to accomplish than a 12v cause, at least in my area, 12v's are going for a lot of money, about double what I paid for my 8.1 with only 91k miles on it, and there is more fab work to get them installed (or you can buy the really high priced kits that make them easier to swap). Plus, I factored in the long term maintenance of both options and track record shows how much less you will spend on an LS based motor (even though the 8.1 isn't technically an LS, they have proved to go well over 200K miles on them with little to nothing done to them just like the 5.3/6.0 motors) as compared to a diesel. Though the 12v is a bad a$$ motor and can go for a long time too, most everyone says you will usually pay a pretty penny each time it goes down for a repair (usually).
Anywho, I just wanted to give you my experience so far since we have pretty much the same truck and you were contemplating the same things I did with what do to with the powerplants. I think the 8.1 is a great motor for the couple time a month tow guy like some of us. And since like me, you will not DD this truck, I don't think gas mileage should be your overall goal IMO. Cause the way I looked at it was I could pay about ~$4000 more to swap in a 12v (once again, looking at the prices of everything in my area) just to get maybe 5 mpg's more overall with the truck. Since I won't DD this truck, I calculated it would take me about 7 years to regain my cost of the swap back in gas money with today's gas prices (yes, I'm a weird accountant that enjoys calculating crap like that). To me, that didn't make too much sense.
Sorry for the long post, but just thought you might like another perspective
So, don't know if you have seen my build to my crew cab yet or not (link in my sig), but I went through all the same processes of you when deciding what drivetrain I wanted to swap into mine. I looked at 12v's, LS's, 4l80e's, Allison's, pretty much everything. I decided on an 8.1/NV4500 in place of my 350TBI/TH400. Obviously my situation may be different than yours and I may want something different out of my truck than what you really want, but I found that swapping an 8.1/NV4500 into these crew cab squares is about as straight forward as you can get (even easier than a 6.0 since you don't have to worry about conversion motor mounts and exhaust is a little easier to deal with). Also, it was going to be cheaper/easier to accomplish than a 12v cause, at least in my area, 12v's are going for a lot of money, about double what I paid for my 8.1 with only 91k miles on it, and there is more fab work to get them installed (or you can buy the really high priced kits that make them easier to swap). Plus, I factored in the long term maintenance of both options and track record shows how much less you will spend on an LS based motor (even though the 8.1 isn't technically an LS, they have proved to go well over 200K miles on them with little to nothing done to them just like the 5.3/6.0 motors) as compared to a diesel. Though the 12v is a bad a$$ motor and can go for a long time too, most everyone says you will usually pay a pretty penny each time it goes down for a repair (usually).
Anywho, I just wanted to give you my experience so far since we have pretty much the same truck and you were contemplating the same things I did with what do to with the powerplants. I think the 8.1 is a great motor for the couple time a month tow guy like some of us. And since like me, you will not DD this truck, I don't think gas mileage should be your overall goal IMO. Cause the way I looked at it was I could pay about ~$4000 more to swap in a 12v (once again, looking at the prices of everything in my area) just to get maybe 5 mpg's more overall with the truck. Since I won't DD this truck, I calculated it would take me about 7 years to regain my cost of the swap back in gas money with today's gas prices (yes, I'm a weird accountant that enjoys calculating crap like that). To me, that didn't make too much sense.
Sorry for the long post, but just thought you might like another perspective
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