Been there, done that, got the T-shirt. In converting my 69 from a SB heater to a BB heater, I got a BB heater core from Autozone (good and cheap), I had the heater box out of the car. You will have to modify the little metal spring clips that hold the core in place. Since the BB core inlets/outlets exit the blower box near the middle, I got a seal kit for the BB from NPD. Once the core was in place, I positioned the SB blower box over the heater box and lined it up as if it was installed. I marked where the new BB heater core inlets/outlets exit and drilled holes using a 1" hole saw. I then opened up the area between them and smoothed it out. I got an "adapter" from NPD (a sheet metal piece that goes over the inlet/outlet holes in the blower box and screws in place). This adapter is a stock BB piece. The new seal kit has the rubber seal used in this adapter and to seal the old holes from the SB core. I installed the new seals in my heater box, installed the core, and it all fit well and looks stock BB. I must caution you to make very sure your blower box is properly aligned on the heater box before you drill for the holes.The whole setup cost me less than $75.00 and looks stock. Very stock BB.