I'm thinking about a new GMC with a Duramax or a 5.3. Don't want a 6.0 or 6.2 because of poor mileage. Currently driving an old F350 with the 7.3 IDI and a banks turbo. The old truck has issues and it's difficult to find anyone who can fix them without breaking something else. Anyway, we pull a horse trailer usually with one horse. It weighs a max of about 5,000lbs. We live in a valley and have to pull from about 3,000ft elevation to about 7,000ft for some rides. Would the 5.3 gas engine pull uphill easily at 70mph or do we need the duramax? I know a smaller engine will do it but I don't want to be creeping uphill with the 18 wheelers or do damage to engine or transmission.