Glad to chime in on this as I have done many various stereos in my 01 bullitt mustang. From mild a sundown sa-8 on 500w to two dc lvl 4 12s on 3k. Now its a street/strip car so no more bang but there are some critical details to doing a mustang system. The trunk opening awkward and the trunk height is around 14-16inches at the highest point, and if he still has the mach 460 like a lot of the mustangs the height is a meer 11" behind the seats. my favorite setup was 2 skar audio 10s on 1500, but anyways. I would definitely go with an audiopipe 1000 monoblock. They are decent amps for the money and I have owned 7 of the 1500s. Check out the big 3 upgrade for a little more input juice as well. It cost about $20 if you order standard 4ga wire and can really help. I just checked ebay and the 1500s are going for $125 now so you might as well just get the 1500 and run it at 2ohm for a little less power and room to upgrade down the road. To go along with it I would either go with a skar audio vvx series either 10 or 12. In a ported box you would be amazed at the performance of them. Do not for a second think that 1 10 wont be enough. It will sound better and have more output than most 2 12 setups that all the kids run in their hondas. with a 10 you can build a box that will fit under the shaker setup and have the port and sub facing forward. I had great results with that setup.
like others have stated the stock speakers are pretty good when you run a high pass filter on them.
