Gears will make the most drastic difference for the money. Don't waste your money tuning a stock 5.0 though, set the timing to 14 degrees advanced and run 93 octane.
After that, FULL exhaust including headers, they are trash on the 5.0 pushrod cars. Also get a set of Steeda subframes installed. Not a power mod but it'll make the whole car feel so much better and firmer. Also lowering is mandatory on these cars to get rid of the ridiculous 4x4 stance and godawful stock handling.
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 ****.