What rear end do you have in your 69? Are you wanting to install just the gears or swapping the entire rear? Your's will have from the factory an 8inch or 9inch rear where the gears are mounted in a seperate carrier. The 8.8 rears have the gears mounted in the rear housing GM style. As far as I know the gears are not interchangable between types.
Not unless there is a 8.8" rear in the car now. If it hasn't been changed, then it is either an 8" or 9" like RacerX said. If you look under the car from the rear and the back of the differential has no acces cover, then you don't have an 8.8.