so the modern hd 6.0 ls engine is the l96 or ly6 one has flex fuel one does not is only difference. they are vvt motors like the coyote and good torque motors. the only ones you really want for a hd truck. again expect 13 mpg at best in stock form in a heavy truck. maybe a tune rv cam e fans and headers high flow exhaust would see a few more mpg but again you are 2 grand in parts on top of the stuff you need to swap. 4 or 5 grand would go along way in a 460. further more what advantage are you gain by spending alot of money to get few mpg better. old saying 4k buys alot of gas. currently a real lot since its under 2 bucks a gallon. you going to be alot more work if you want a stick behind a ls in a hd truck. you could find a older gm hd 6 speed would be about the only option. i know the pre 07 up trucks had an option in some years. not going to bolt to many 5 speeds out there. on top very few are going to hold up to the abuse of that kind. so i owned a 07 first year of that body style cc lb ly6 6l90e 4wd truck. i hauled 4k in the bed more than once. i also pulled several cars. it did down shift loaded more than the duramax 04 i had but i had not trouble pulling up hills at the same speed as the duramax with similar loads. honestly i prefer the gas trucks to drive daily when not loaded. my cummins i have now i hate on the highway.