Coupla things:
Somewhere in the archives of
The Diesel Page - for the 6.2L, 6.5L, and Duramax 6600 (6.6L) GM Chevrolet and GMC diesel engines. there is a story out of GM about a 6.2 normally aspirated diesel with a 700R4 and 2.73 gears that got 30 MPG without hypermiling. IMO this thing would be so slow any driving it would be virtual hypermiling. Look for the story.
Back in the 80s and 90s I had a 6.2 diesel with a 700R4 in a K2500 pickup. Not much heavy work unless you count hauling my fat butt around. Good truck - got about 23 MPG but went through transmissions like a teenager goes through potato chips. By time I got rid of it at 305,000 miles it had trashed its seventh transmission. spectacular failures with parts and tranny fluid scattered down the road.
My advice to a good mechanic: Transplant in a manual. NV4500s fit well. T5s are too light.
Also a manualwould enhance MPG.