my memory is really shady in this area, so someone correct me if I am wrong. My memories are also based on 1999 data. There are at least 2 styles of operating a fuel injector. One of the ways pulls more current than the other, therefore if the mustang uses the lower amp injectors installing the higher amp injectors would fry those transistors in the computer...something to think about.
If the injectors are however of the same style then there is no reason you couldn't use an injector just because it is rated with metric units. For example, using your formula, the stock mustang injectors flow 19lb/hr *(10.5cc/lb/hr) = 199.5cc/hr