From the research I've done; the better lifts wont go over 4" because they feel that lift brackets that deep are too weak, instead they cut and turn the beams to gain lift. ie. desert racers, prerunners.
The notch is probably clearance for the pumpkin, but not having done one , am not sure. I've seen pics with problems clearance between lift bracket extension and pumpkin when reusing original pivot bracket. Most but not all aftermarket brackets seem to have been designed to take this into account.
Your b-II lift springs probably won't provide as much lift in the exploder.
Richard