Yeah, 2 is funny, but 3 is very practical.
The benefits of putting a 5.0 in a 4 cyl body is that the body is probably in better shape because it isn't under as much physical stress. 4 cyl Mustangs are also a lot less expensive than 5.0s. Why pay a lot more for something you plan to replace with the engine you have? If my frame were to crack, or my torque boxes were thrashed, I would probably just buy a 4 cyl LX and move all of my performance parts over.
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351W 89 Mustang GT Convertible