Fast + budget are not synonyms. If you want to go fast, its not cheap (especially with a 5000 lb truck).
Like Kay9 said, spray would be the best option for cheap speed, but without supporting upgrades youll be severely limited in the shot size you can run.
Id personally suggest a tuner ($300), set of headers ($500), used CAI off forums ($150), new y-pipe and muffler ($200 installed for both if you cant weld them in, $100 if you can), and a cam ($450 w/ cam/springs/ect). That would get you breathing quite a bit better and combined Id estimate around 50 whp.
with 1500$ unless you use nitrous nothing will get you faster that a 2007 mustang gt
buy a 5$ sugar pouch put it in his gas, and then go race,... you might have a chance
but if you whant to mod for that money i agree with doz + i would had with everything done including the cam, a custom tune , with would be from 200 to 500$ but would bring everything togeter and instead of 50rwhp you might get a another 15 to 25 to rwhp buy custom tuning with everything including a cam. but you wont be faster tham a 07 gt , might be a little close but not faster
also if you dont have them, put 3.92 gears,... i would buy them in a crash/scrap yard from a damage ram
Yeah telling guys with Mustangs you wanna hooks chains usually shuts them up. As far as Mustang being faster I dunno bout the newer ones but the late 90s early 00 I've never had an issue with in my slightly modified truck. Only one I have an issue with is my buddy's supercharged 99 Mustang.
I would suggest going out and buying a 360 cu block and boring it out to a 408 in the long run your gonna be spending more money on the motor and all new mounts. Plus the motor will be worth more than the truck, but your dad wont ever see anything but your truck in the distance