Hi Joe...
the mustang swap is quickly becoming a popular swap here on the forums. The engines are nearly identical aside from the bolt ons like the intake, fuel injection and accessory drive parts. You can only use the "Long Block" from the mustang. Everything else has to come from your explorer engine. Aside from that, it is pretty much a direct bolt in swap.
The antenna... I can't use the area over the front seats since I have a sun roof. I will be mounting the antenna directly over the rear dome light with the radio mounted in the rear compartment on the driver's side... probably on the sheet metal over the fuel filler neck. The face/control head will be mounted up front where I have easy access to it, and a speaker will probably be at home on the side of the center console.
The Wilson antenna you show in your picture is a nice antenna and is a great choice for a CB installation, but my radio is not a CB, and so the antenna requirements are quite different. My radio is a UHF/VHF FM radio, more like the types of mobile radios the police and fire department use with repeaters.
If I were to ever install a CB, I would certainly agree that the Wilson is a great antenna.
BTW. Where did you mount your radio? Do you have any pics?