I would do a 6l or 7l. I think the 6l would be ok cause your batts can handle it. It will draw some current and make quite a bit of power. Plus it is cheap, not sure if Mike has any laying around, but Steven at offshore electrics can hook you up. 60 bucks and about 5 or 6 for shipping. He uses that same shipping method as Mike, the fast one that no one else can figure out! He is a great guy, will go out of his way to answer any ? to the best of his ability. He is like the electric boat version of Mike!!
Heres a link to his site, check out the boat hulls, he has some killer stuff. If you could not tell I am also a brushless boat freak too, Nothing Like 40+ on the water in a 27 inch boat!!
www.offshoreelectrics.com