If you're going for a dsl swap "just" for mpgs to save money That's generally not a viable economic plan. By the time you figure in total: buy-in eng costs, freshening up costs, accessories/mods etc costs, maint you're in it for thousands. You need a "sharp" pencil to figure how long/many miles you'll need to drive out the money spent just to break even or just gradually spend the money on gasoline. You heavy towing, doing mega long freeway trips? I haven't done the swap game...numbers never added up for me. I've had a couple 6.2's in their stock trucks and for work and mpg efficiency they like the 2000 RPM range. With a stock non-OD trans I've found 4.10 gears to work pretty well. Low to mid 20 mpgs avg all round use (rural, hwys etc but not towing or city traffic). With your crappy 85GW stock axle gearing you WILL need to re-gear your axles...$$$. Good luck with your "sharp pencil".
"Don't mind me. I'm just here for the alibi"