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Ditto on the Delco alternator, this is what I did nearly 3 years ago on my M925A2 and was able to use the same length serpentine belt by changing the length of the lower bracket.
http://www.steelsoldiers.com/showthread.php?85317-M925A2-Delco-Style-Alternator&highlight=m925a2+delco
Check it with a digital VOM and see what your readings are. Load test and check each battery individually while still in the truck, no need to yank them out unless 1 is bad.