The rear in the car is an 8 1/4, not 8 3/8- thats a later truck rear, since it's a 69 I would look for an 8.75 because you will need it anyways if anything fails in the 8.25 because there is nothing available for it 69-71 or so are different internally from the later ones and parts don't swap.

I found this out years ago with a 70 coronet I was working on, it was some highway special with a 383/727 and 2.76's in an 8.25, I was trying to help the guy build it on a budget and gave him the 3.55 sure grip out of my '73 Duster to swap in and it wasn't close to working.

If a stout 440 is going in I would skip the 8.75 and go with a Dana, it's stronger and about 1/2 the price of 8.75 stuff anymore.