Span is rather irrelevant, the only thing it might influence, depending on the pin distance of the ball, is the final drilling options. Sometimes, the pin might end up very close to a potential finger hole, and you either have to drill it through the pin or leave 1" space to avoid breaking.

Pin placement is primarily based on the PAP and the bowler's style (speed revs, tilt, as mentioned), and you will IMHO also have to consider coverstock material properties and surface prep.

3 3/8" from PAP ist the most instable position for a core, with most flare potential. Closer to PAP, the core is more stable and the reaction more rolly, further away you also have less flare and hook potential, but a later/sharper look. This defines, more or less, the hook shape.
Pin close to the grip center (normally under fingers) will have the ball react earlier and less "violent", further away (above the fingers) will delay the reaction and create length. Finger hole position might not permit every layout (see above), but these are the basics.
To keep the ball legal, the CG is t normally to be placed in the grip center area. Therefore, a long pin distance is quite suitable for length layouts with pins above the fingers. With a short pin distance you might run into static weight problems (high finger weight) if you want a certain reaction out of the ball. A good driller can cope with this, but if you know what you want a ball to do, you can select one with matching specs and exploit the ball's potential within legal limits more than any ball and forcing it to do soemthing it was not designed to do or for what it is not really suitable. You will, for instance, have some problems to get a pin-in ball (say 1" or so) to be really long and flippy - compared with a higher pin distance. The differences might only be marginal, though.
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DizzyFugu - Reporting from Germany
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