2008 Hummer H2 serpintine belt

High output alternators generally have a larger pulley on them that would explain the fit problem. Compare your alternator(o.e.) pulley size with that of the H.O. The next thing is finding a longer belt, the part number of the OE belt will correspond to a size in millimeters, and you can add what you think you need by looking at similar belt specifications. It will take some trial and error. A parts supplier should be able to assist, they will have more options. A dealer may be able to give you the specs on OE belt, but may only have belts for specific applications.