I believe you're asking the wrong question. you won't gain any performance from full length stringers themselves. My 18' Hawaiian has stringers that end where yours do. But under the floor the are 2x4 "stringers" that are glassed along with the floor to provide stability and rigidity to the hull, they form a rigid box. With stringers, you should ask, how much strength will they provide? I think the problem on our boats is over time hook can develope in the area where the stringers stop and the floor begins, about a 2" gap in my boat. If you're going to run a standard amount of hp, up to 500 or so, it shouldn't be a problem. Over that the motor could be putting too much stress on the hull or so I've heard. Otherwise, you should be ok, provided the floor is good and you don't already have hook in the hull. My boat is a 1976 and the bottom is laser straight, as they say.
All of that being said, I would look for full length stringers to be in my next boat, I just wouldn't bother trying to attach them to my hull. I have already replaced the 2x4s and floor.
Just my .02
RTJas http://free.***boat.net/ubb/biggrin.gif