David, look closely and you will see those same lines at each corner of the main and jib. It looks to me like the material in those areas has a different bias to spread the load in the corners. The curvey lines are the borders of the different materials.
I, too am resurrecting a 5.0. I'm not sure if I'll finish by the end of the season, but I'm getting close. I got new sails, which I have not gotten up yet either. I'd love to compare notes as we go along. I got a lightly used jib through someone on the catsailor forum, but I could not find a used main after a few months so I went ahead and bought a new one from Skip Elliott at http://www.epsails.com/
Before you think about putting a 500 sail on a 5.0, talk to the folks at Performance Catamarans (Nacra). Bottom line, they said you could physically do it, but it would not be optimal. The boat dimensions are different. Call them & talk about it. I think Bob (?) was the person I was referred to there.
Good luck!