Glad to see another CarPC in a Mustang, I've got a '68 I'm in the middle of right now. Love the early style Shelby hood, I've been thinking of getting one myself for awhile.
As for mounting the PC, might I suggest taking the rear seat out, and putting a piece of sheet metal between the trunk and the interior, did this myself and it makes the perfect place to mount to.
Once again, glad to see another Mustang, and great looking car.
EDIT: You might want to look into Lokar for some cup holders, they sell just about everything for street rods, and have great cup holders.