Msd pro billet in a small block 360 backfire at idle

what i have found with the msd blaster coils is most of the time you have to switch the +positive and -negative wires at the coil, unless running all msd components for ignition. i know it doesnt make sense. i believe that is because a msd box trigs the coil on the positive side of the coil where as most other boxes trig on the negative side, which is what we are used to. maybe you should try it.
most of the time, you should use a msd coil with an msd box. but use another brand coil with other brand boxes.
you might want to try your old coil to confirm