Is it attached to the house? If it is, is the shop on the outside end or between the house and the other stalls? I assume you have two drivers and the project car for three cars total and that is why you need a third stall?
I guess it depends on what kind of work you have planned, how much room you need, how often you plan to use the extra car, what kind of access you want for it. If you decide you don't need the shop space for a car then I'd keep it intact for engine building and parts storage.