Stock manifolds flow just as well as shorty headers do. They look like crap but... eh... Unless they are broke use that grad money on something else. Put up a mod list and we can see what you don't have that would actually help you out more than longtubes. Also longtubes are a ***** to install on a modular. Not like a 5.0L where you can install them in a few hours without ripping out the K member...
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.