If they are flared, they are more efficient than tubes, which are still more efficient than slot ports, 2 6s should be fine. Remember, there are rule of thumb for sq in of port but truly you need to base it off of cone area and displacement. I used 3 4in for 1 15 and it did great. I might go with 3 6s if you could make it work, but I think 2 6s will do.