I don't remember the exact number, but ...
19# injectors are good to about 270 - 300 HP.
24# injectors should cover you to for 350 - 380 HP
You can put the 24# injectors in a stock engine, but you are not using the injector as they were designed, so you will not get the same spray / atomization with 24# injectors in an engine that only makes 250 HP. Also remember the HP numbers we like to use only represent a portion of our driving. How often are you running at 5600 RPM making full HP? In most cases the 19# injectors should work out fine. Search for "mustang math" on the web and you will get some formulas you need to address this.
On the other hand if you put in an aggressive cam, aftermarket heads, do some porting, run a larger TB & MAF with headers. And you like to get on it ...
Go for the 24# injectors. You might also consider a Cobra ECC unit (fuel curves for 24# injectors)