This thread provides more details than you need for just a belt replacement, but should give you exactly what you are looking for: http://www.peachparts.com/shopforum/diesel-discussion/312419-diy-1987-300d-serpentine-belt-tensioner-replacement.html

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97 S500 (90,000 miles) - wife's car
87 300D (298,000 miles and a replacement #14 head)
94 Suburban K2500 - need something to haul firewood
83 300SD (343,000 miles) (sold)