Like others, I think you'll want 8 total: 2 each at bow and stern, and two spring lines each port and starboard. How long the spring lines are depends whether you have a mid-ships cleat or not. If you have to spring at the bow and stern cleats, you will need longer spring lines.
You might get away with single spring lines, but it will be harder to do in such a big slip.
A 40 foot slip can actually be a bit tricky for tying up a 30 foot boat. We were in a 40 foot slip this past winter, and it was not easy making it work. The forward pilings were almost too far out for tying the bow, and the middle pilings were not in a good spot either.
We have a heavy set of winter dock lines, and then a second set of lighter lines we use when we're in transient slips in the summer (we moor ordinarily). The winter docklines go in the garage during sailing season. If we had a slip, we'd leave those plus-sized lines affixed to the pilings.
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Pacific Seacraft Crealock 31 #62
NEVER CALLS CRUISINGDAD BACK....CAN"T TAKE THE ACCENT