96 K2500, 454/4L80E, on 285s with 3.73 gearing, an 0411 swap and true dual exhaust; after 0411 swap and when I wasn't fighting any issues I got 10-12 running around town and driving on the interstate.
I've gotten anywhere from 8 to almost 14.5 I think, I got that on a long road trip but I think the truck was running really lean; compared to pulling my camper and dealing with a violent misfire got me 10 surprisingly cruising at 50-60 mph
Personally I would expect anything better that 10 or 11 MPG, plan for 8 and hope for higher.