I'm not sure about L300, but we had a hell of time finding replacement alternator / fan belts for our '95 L400... so many of the OEM or replacement-for-OEM part numbers are dead-ends.
So after hours of research, trial-and-error, we finally found the Gates belt that fits our 4WD 2.8L Diesel L400 Space Gear: Gates 7385.
It's a grooved V-belt, 0.41" wide with 38.51" effective length. A Gates 7384 may also fit.
Once we finally located them, we bought 4 just to have an extra set. These are high performance V-belts. Don't get fooled into using a low-horsepower lawn mower belt just because it fits. Those thinner belts will melt and fray at high temps, and the alternator won't be able to fully charge the battery.