that is a VERY vague question. if he is close to stock or mildly modified, you will win by a huge amount. some 5.0 stangs will KILL you though and leave you behind by buslengths. it also depends on if you race from a roll or drag. if he does not know shit about Supras, you will most likely kill him. if he does and still wants to challenge you, you might be in for a big surprise. well, get some more INFO about the stang before you ask any more questions. and good luck.
(5.0 stangs can be anywhere from 215horsepower to over 1000...to give an example of just how umm...senseless...your question was)