Ford built the plant, sold it to Mazda and then bought half of it back as joint venture.
One of the stipulations about the GT-500 was it couldn't take any longer to assemble than a comparable GT.
I am sure there is a rhyme or reason to the way that they do it, but it is weird to see very different cars rolling down the same line. Normally cars that share a line are somewhat related, like the Ranger/Bronco II. Right know they seem to be concentrating on the V-6 Mustangs, a local dealer has one sitting on a lot somewhere up there waiting for the '11's to be officially released, but the GT my brother ordered in February has yet to even have a production date set. They haven't ordered a '11 GT for the dealer yet, so maybe they are only working dealer stock orders first before they do the special orders.
I don't know exactly what they really make in house at the plant. The Shelby 5.4's and GT 4.6's are/were made at the Romeo plant, and the 4.0's are made in the Colone, Germany plant. The 5.0 will be made at the Essex plant (dunno if they have started making them yet??) and the 3.7 is made at the Cleveland plant, so they don't make the Ford engine at the same plant as the car. The transmissions and I would assume axles are also made at an off site.
One of the things about the modern production theory is that the required part has to be showing up just about as the guy needing for it is reaching for it. A storm holding up production or having a years worth of them sitting in a room is not a very productive way of running a factory. It isn't all that likely that minor hiccups in deliveries will effect production if things are set up properly. It is a tricky thing to do, the import guys pretty much mastered it (thanks to an American) and proceeded to use it to kick the Domestic's butt until recently.
I went thru all that crap in college.