5.3l into a k20 or k30

[chevyman95]

What do you guys think about this swap? Will it be too hard on the motor? I can get a 5.3l relatively inexpensively from a friend. I wish it was a 6.0 or 8.1 but for mileage concerns the 5.3 may be the best bet. I don't really tow that much weight just occasionally one car trailer. I'm looking at a k30 right now that has a 454 in it but the owner says it gets about 8mpg which wouldn't be very doable for me in the long run. Looking for 12mpg, diesel swap is out of the question for me right now. Thanks