I dealt with this problem recently, and it's easy to address. The factory speakers are actually 8ohm - higher resistance - and Ford did some funky wiring where the two front speakers are effectively wired in parallel to the amp and the effective resistance of the two front door speakers is 4ohms at the amp. I've read the stock Mach amps work well in the 3-4ohm range.
Most of those Crutchfield lists will correctly fit size-wise, but they're mostly 4ohm speakers. When two of those are put in parallel, it gives you an effective resistance of 2ohms. If you've got the volume cranked up, the amp will periodically cut out to cool off, as the lower resistance is drawing more current (at the same voltage) and overwhelms the amp.
A lot of people suggest bypassing the amp and installing their own, or hunting down expensive used 8ohm speakers. I'm cheaper than that.
I picked a set of speakers that had dome tweeters separately wired to the connectors. I actually went with these from Crutchfield -
http://www.crutchfield.com/p_206DSC504/Kicker-43DSC504.html?tp=105
If you take a look at the back, there are small wires soldered to the back of the connector leads. Those are for the dome tweeter. I chopped them with a pair of scissors, and measured the speakers with a multi-meter, effective resistance ~7ohms instead of the ~4ohms I'd previously measured (note that this isn't an exact science as this is not DC voltage coming from the amp, and various frequencies are filtered).
What I can tell you is - the current speakers have no tweeters (those are near the pillar on the door), the amp probably doesn't send high frequencies to the door speakers anyways due to its internal crossover, and I can crank the radio at full volume all day long without the amp cutting out. The speakers also sound really good -- especially compared to the crackling, dried out ones that were there.
So if you don't mind a little DIY (30 seconds or less but certain it voids your speaker warranty), you might find this also works for you (the only reason I say 'might' is that I have no idea what your head-unit looks like), and retains the factory Mach 460 goodness while being an inexpensive work-around. The amp's expecting an ~8ohm woofer at the end of the wire, and that's what we've given it.
