The proper installation, if you really want to use a separate engine driven pump, is to have the pump connected to the splined end of the engine crankshaft by a short shaft with a flex coupling in the middle to deal with any misalignment issues.
Finding the correct splined fitting is a challenge.
Here are three poor quality photos of the type of pump drive I am referring to.
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It is more common to use the tractor hydraulics to power a loader. You may loose a bit of speed but the convenience of not having to maintain a separate hydraulic system with filter and hoses can be attractive.
Do some searching of the forum for posts by rbargeron on the subject of hydraulics. He knows the old L's very well and his posts may contain the hardware necessary to draw hydraulic power from the existing system.
Look on kubotabooks.com in the loader section for material on the 1200 series loader which was the model intended for L series tractors.
kubotabooks
Found a better pump drive photo.
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Dave