While Dave is right about the issue of too much rigidity being a bad thing I realy can't see the extra 1/8" making a whole lot of difference to the rigidity and therefore not an issue as far as a Curlew build. As I mentioned before Jeff's plans for the FreeB 12 say either size is fine so the issue of stringer flexibility is not too critical here as the curlew has loser curves (depending on the wood used of course). In my Mess About 12' build I was abe to bend spruce quite easily for the stringers, gunwales, and keelson. I think what is realy the important thing is that the boat be lashed together well and skinned tightly.
Having said that, if you can find a souce for the 5/8" material you wouldn't have to mess with changing the plans at all which is always a bonus!
Enjoy your build!