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)

This is a great thread.

The real trick is to use the handle of the lug wrench to relieve the tension, once you get the nut/bolt loose.

Probably my least favorite thing is getting that fan off, but boy is life easier once it is out of the way.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC