Engine change from a 302 to a 5.0 efi?

my son put in a 5.0 engine in his 69 mustang. he had a old carb. 302. now the 5.0 motor wont idle smooth. it was a speed denseity set up, now a m airflow system. the computer is from a 1991 mustang complete with all the engine harness. the cam is pretty much stock . no big lift or duration.he used the mas air flow from the oem motor (1991) . it seems the motor runs real rich and sounds like it has a 600 lift cam in it at idle, you can smell it. he also used the cats and o 2 senors. his car runs good,other wise meaning he can get rubber in all 4 gears !! there are also no codes being logged . thanks for some info about what to do next. navy dad