get a large sheet of stiff paper. measure the ID of the register hole on the new BBP drums. stick the point of a compass in the paper and trace a circle the dia of the BBP register hole. then trace a 2nd circle of 4.5" dia. then make a pinpoint prick with the compass at any place along the circumference of the 4.5" circle. then make another pinpoint mark along the circumference 2.645" but in a straight line tangent (2.645" in a straight line) with your ruler. repeat (3) more times. drill these (5) pinpoint prick marks out to the dia needed for 1/2" studs. use this template to locate the new 4.5" BBP holes in the new SBP drum (if I am reading this right). the BBP center register is larger on the BBP drums so the center hole will need to be enlarged (if needed, did not read all the way thru so see what you are going with). FYI SBP (7/16) holes would be 2.351" along that same straight line tangent working with a 4" dia circle & to confirm, that distance is center to center of the holes from the pinpoint prick marks