Chris,
Sounds like it should work with plenty to spare. The Mustang II is around 5'9" wide and about 50" tall. Since the rotisserie is going to be mounted in the middle (from drivers to passenger side), you should clear the ground with the car tilted sideways by about 23", flipped upside down, I'm guessing you should still have at least a foot of clearance
Is it too tall? I'm guessing you will be mounting it at the frame stubs in the front and the leaf mounts at the rear? I'd rather have 1" of clearance than 2'...a lot more stable
Sounds like a great idea, wish I had my car on one right now.
Francis