The Duramax/Allison is outstanding, and even better if you add the Bully Dog computer chip. Just drove mine home from Tennessee to Alabama this afternoon, and got 25.6 mpg at 65 mph with the bully dog turned up about a quarter turn. Awesome power and performance in my 2003 Silverado 2500HD.
As for putting a diesel engine in a former gas truck--I think you'll find there are some major suspension changes that will have to be made also. The diesel is way heavier. And yes, it is about $10K more than the gas engine when new.