#1. Despite what you claim, your 6500lb truck with 4.56 gears will NEVER get the same mileage as a diesel with 3.73 gears. No way! No how! It would defy the laws of physics. There simply isn't as much energy in gas, and it doesn't burn as efficiently.
#2 You keep talking about cost. Your truck cost MORE than a diesel. Why does this escape you EVERY time it's brought to your attention.
#3 It's a M-Fing Gas. IT WILL NEVER last as long as a diesel. You see diesel pickups all over the place with 300,000-500,000 miles. The same truck with a gas engine MIGHT last 300,000 if you're very nice to it AND get lucky. A well designed and properly maintained diesel engine will always outlast a well designed and properly maintained gas engine.
#3 (a)-Will your truck last 30 years? If not, why the F**k did you bring it up? (b) There are poorly built diesels that are over 30 years old(6.2 Detroit), without a rebuild. Lots of 6.9 Internationals that are coming up on 30 years old. Explain why EVERY real tractor build in the last 40 years is a diesel. It ain't because they pull worse and are more expensive to maintain. A diesel and gas truck share the same body and frame. It's been well established that a diesel will last longer. The only case where a diesel caused a truck to wear noticeably faster(front end) is the Cummins in the 94'-06' Ram.
#4 Everything comes down to math. Let's assume that your truck has 500 lb ft of torque and a diesel has 800 lb ft of torque. The diesel is multiplied 3.73 times through the rear end. Meaning the axles are transmitting 2984lb ft. of torque to the rear tire. The gas is multiplied 4.56 times, to deliver 2280 lb. ft. of torque to the rear wheel. Diesel wins even when the gas has an exaggerated amount of torque. Lets try that again with 560 lbs. ft. of torque for the gas. (an additional 153 lb.ft. of torque over factory) 560x4.56=2553 lb.ft. of torque through the rear axles.