Just about the only common parts between a 4 cylinder Mustang and a 5.0 is the body and upholstery. Almost everything else is either 5.0 or 4 cylinder specific. You end up buying two cars and stripping the 5.0 car for parts to put on the 4 cylinder car. Not worth the effort unless you get both cars in excellent shape for $500 a piece, and have a shop with all the tools needed for a complete tear down and rebuild.