You don't need to join the area where the two carbs go---just machine the manifold flat and build a plate that matches the blower base with 2 holes for where the carbs would have been---the phlemem area does need anything---if you are concerned you could put a divider left/right side ---the blower turns outwards so would work--the least amount the manifold is divided the better it will work

need more low end torque push the pedal down further