Swap to a 6.0L with a 4L80e. That way you'll definetly get some gains in hp/tq an LS engine, and less mpg's lost with driving around that heavy tahoe, plus less mpg wasted when towing or carrying weight, and probably an overall increase of gas mileage maybe putting you back into the 16-17 overall mpg range. Don't go down a pipe dream of putting a 6.5L into an obs tahoe its a waste of money a total waste of cash just to get a little better gas mileage, some black smoke, and the noise, vibrations. I'll admit I LOVE The black smoke, the noise, and the gas mileage and the torque from a modern diesel engine. But this is NOT the right route to go. Your MUCH better off trying to find a 2dr tahoe with a 6.5L diesel and buying it and driving it or finding a 1500HD with a 6.5L buying it and driving it if they even made those if your looking for something 1/2 ton with a 6.5L and the old body style. Hey and if you do find a diesel 2 door in your search, tell me I would love to have one. Also an M1009 with a 6.2L is not what your looking for they are tin cans.