What are the rest of the specs? Many alignment places are really not very good at this stuff, if it isn't in the computer they don't have a clue. I am asking because I have to wonder could the camber be lessened to give you the caster? Could a better happy medium be found in other words.
Do you have photos of the suspension? How about the locator/strut bar, does it have one (some Mustang II front ends are modeled after the original car with them some aren't) or does it have a full A arm that eliminates it? That bar can be adjusted to change caster if it has one.
Does it have adjustable lower control arms?
I am amazed at how some "pro" alignment guys don't see where adjustments can be made when it's not in the computer! We have an alignment system at work but have to farm out the alignments once in a while when the rack is tied up or we simply have too many to do. We have a very well known shop near by. It has happened a couple different times when we are told it won't align, sorry there are bent components. I go over there and have to show them where some point is adjustable that they didn't even see! I was told "Hmmmm, it didn't tell me you could adjust that" like I'm making it up or somehting.
The guy may be looking at something very simple but not seeing the forest for the trees.