[:-)]

Do they want the exactly shape of a surfboard in terms of the rails, for example? If they want to use it as a shelf I would almost suspect that planing the rails on one side would be best and making the rear flat so there is more of an edge against the wall. If they want you to basically build a 5' surfboard then you'll cut and shape as normal.

I've taken part in one surfboard build for riding not for shelving. There were templates and such that were made to get things right. I was under a lot of supervision by a board maker.

Here's a site I found when i was trying to look for a book that was once recommended. You may find it useful.
http://woodenlongboardplans.com