Not hard, just time consuming and probably not worth the effort. You'll likely need new motor mounts, a tranny adapter, new exhaust manifolds, accessory brackets and a few other things that haven't immediately leapt to mind. You are also not likely to see a big difference in fuel mileage and certainly not in performance. Still, I'd be curious to hear the results from someone who's actually done it.
IMO It is not worth the effort,you'll need custom engine mounts,different trany and t-case combo,unless you spend the bucks for adapters and all that stuff.My 1/2 ton to came with a 302 in it it was a total butch job.Truck was way underpowered,had a c6/np208 combo which really downgrade the entire truck.It eventually got a 351M back in it with a stick and 205 t-case.