Considering the deck height is taller on the 351, are you surprised? If you're using a hydraulic roller cam, just pick up a distributer from a '95 ford pickup. They're cheap and work great.
If you go the boneyard route, trucks/vans are good source because they did not get EFI engines until a decade or so later after passenger cars. Can't use the distributor from an EFI engine...and even later there is no distributor.
In my mind, however, regardless of what distributor you get, you need to make sure the centrifugal advance and vacuum advance are properly set for your car. Settings such as:
1. At what RPM does the centrifugal advance begin to take affect.
2. What is the relationship between the vacuum and the timing advance of the vacuum advance?
3. At what RPM is the advance set to its maximum.
4. What is the vacuum advance maximum.
I do have the specification tables for the 73 Mustang, but I don't know if they include the distributor. The tables were in a supplement to the shop service manual. I will try to post it this weekend(or after a prodding Monday).
So, if you do get a distributor in their, whether boneyard or your local auto parts store, don't expect it to work as well as it COULD. I can GUARANTEE you it was not set for your specific application. It was meant to FIT and that is about it.
I am sure there are plenty of opinions regarding the 4 settings I mentioned above...just as there are plenty of opinions regarding carb jet sizing, carb CFM size requirements, blah blah blah. Makes you wonder how Nicolas Otto ever got his first 4 stroke engine started.