I can only comment on '67 BB's. I haven't seen any FW differences between SB and BB (heater core cover exception), but the motor mounts for BB's are different in that the motor is shifted over approximately 1" to the right. Plus, the transmission cross member was unique to the BB to accommodate the motor/transmission offset. Again, this is for 67, but the '68 should be close, if not the same.
Mike