changing sbp 8 3/4 to lbp

The backing plate is different for the BBP brakes, and it was never available with the 8.75 in an A Body, so you'd be looking for either a complete B Body set-up, or later 8.25 A Body. If you do change the backing plates and drums, you've still got the small register on the A Body axle flange, so the drums won't center correctly. Just drill what you have. Or, sell them and buy a pair already drilled (Which I happen to have..PM me if you're interested)