I put a 65 posi rearend in my 69. Coil on the 65, coil on the 69, what I found was the 65 diff was about 1"-2" narrower on the 65 diff on each side. worked out good for me because I have custom made 12" wide rally rims( to fit my 50s), with a lot of offset to the outside. If I kept the 69 diff the tires would stick out past the side of the box, this way they are flush Gives it a nice look too.
MY2c