Well, whether he's confident he'll survive and whether he'll actually survive are two different things. I don't recall off the top of my head how often Zouken needs to get a new body, but the time spent in each one is getting shorter and shorter. Although you do have a much better recall for these factoids than I do, so maybe you're taking that into account already.
Plus, if he has to make an entire new Greater Grail, I think it's going to take him more than sixty years to build and fill it. Maybe he could salvage the Greater Grail and redesign it for a new leyline. It's questionable though. The Matous were brought in for the Command Seals and binding, not gathering the energy. Zouken probably doesn't have the know-how to fix it properly.
You do have a point about the Association though.