94 mustang problemz

Got a 1994 Ford Mustang v6 3.8 liter, has a true dual flowmaster exhaust installed on the car, gas is pissing out of the left tailpipe, as well as spraying out of the last cylinder on the left side, thought it was a bad fuel injector, changed it still no good, hooked up a fuel gauge got -1.5 psi in the on position and when the car is turned on the guage reads 29 psi, ive never come across a problem like this, if anyone has any advice please post respone thankyou.