Two or three issues with a ford 9" rear end.
First: Width- Since you are going to a new bolt pattern, you are going to have to switch rims too. More modern rims are 'backspaced' differently than the original rims, usually moving the tires closer together. So, when you replace the axel and rims you usually go with 2" + wider than the original, not narrower. Trying to space your rims out 2" on either side by adding a spacer that'll cost you $100-$200 and compromise the strength of the differential might not be the best use of your money.
Second: Bolt pattern- When you change the rear differential you'll get a new bolt pattern for the rims, probably Ford '5 on 5,' which is 5 bolts on a 5" circle. Chevy made a 5 on 5 bolt pattern too. Your real issue is going to be that your front tires and back tires need two different sets of rims, requiring two spares... If you are doing a frame off restoration and upgrading to disc brakes in the front you should try to get a rear axel that matches to bolt pattern on your brake upgrade. Now, you could upgrade the front brakes to match the rear axel, but not in a standard kit. All of the kits I found for my front disc brake up-grade were '4 3/4 on 5.' DOing your own front brake upgrade isn't that hard, all you really need are the brakets, you can buy rotors with a bolt pattern that matches your rear axel and then buy some standard calipers, but that'll require more work by you. If you ain't up to the work, don't get the rear diff.
Third- Cost of rear diff. A ford 9" for $150, if the brakes are in good shape and the pumpkin is in good shape is a pretty good price! At the local junk yard I can pull them myself for $100.
Fourth- Convenience- Racers love the ford 9" 'cause you can swap the pumpkin pretty easily. The pumpkin from almost every ford 9" will swap with every other, so if you want to change the gear ratio its easy to do.
If the axel were the right width and you could make your front bolt pattern match, it would be a good deal. But since its gonna cause you lots of headaches to get the width right and the bolt patterns to match, why not hold out for something that'll work better for the same price?
