After looking at your mods in another thread, just about any cam out there is going to be better for your car than a F303. The F303 is intended more for blower applications.
With the stock heads, you do not need anything far from the stock cam. The stock cam is actually quite a good cam until you really start moving more air. However, if you really are dead set on swapping out it out...
I would recommend the Comp Cams XE264:
1500-5500 power band
But beware! You will need to upgrade your valve springs on the stock heads! These XE cams have extremely quick ramp rates, which plays a big part in how they make good power throughout the power band. But worn out stock valve springs on one of these guys is not a good idea.
The Crane Powermax 2030 is another popular choice for mildly modified Mustangs that won't be quite so hard on the valve springs. It is proven to be a good cam for set-ups similar to yours.
The BEST choice would be to decide on where you want the car to end up before you waste money on a cam. If you are looking at changing the heads out, the cam you buy now will probably be terrible then (or vice versa). The cam should always be the last part you buy. Decide on your heads, intake, gears, etc. and then select a cam that will maximize the potential of all the other parts.
Last but not least, when you are ready to buy a cam, call a custom cam grinder. They can set you up with a grind that will work way better than just some randomly selected OTS cam. You may pay a little more, but the cam is without question one of the most important players in making power. It is not the place to be cheap.