I have been thinking about the same thing. Put a 5.3L in my truck. I think it will be a lot cheaper than the 5.7L (LS1) due to the shear numbers of trucks with this engine in it versus the number of camaro/corvettes outh there. Also would pick up the 4L60E. Ought to be good for at least 20 mpg.
The 5.3L have good performance potential too.
I think the 5.3L mounts are the same style as the LS1s, so you'd need to either fab some up, or buy some aftermarket ones.
http://www.brphotrods.com/parts.html
might call these guys...they might be able to help you out.