1994 Ford Mustang leaks gas out muffler and thick smoke

Hi I have a 1994 mustang n I recently got a new emgine put on because da exhaust came out brote whote n thick so I got a new engine put on but now it still comes out thick n it also leakd the gas out of the muffler
The car wasted 5 gallons in less than 4 minutes and it overheats fast as soon as the car is turned on
My mechanic says its the injectors so I put all new injectors in but still does that. Please help me n I will send u a nice check