technically no such thing as an 86-89 cobra engine. The SVT cobra started in 93. The Cobra 5.0 in the 94/95s is the 5.0 HO with GT40 heads,upper/lower intakes, and some other minor stuff I believe. I would just get a low mileage standard 5.0 out of an 87-93 mustang (easier said than done, I know) and build that up. With EFI and some boltons, they can be very fun motors.
For a sports car-type project, I'm not sure I'd do a 351 (though they aren't that heavy, and would work). A 289/302 would do fine.