By “white line”, Pusha literally means that they stay on the highway when they’re transporting cocaine.
He might also mean that he and his dopeboys don’t get high off their own supply of cocaine.
According to Urban Dictionary, alphabet boys are agents from three-letter acronymed agencies: the FBI, CIA, DHS (Homeland Security), etc. Part of their duty is to go after drug traffickers.

As Biggie (who Pusha is known to praise) said:
Never get high on your own supply
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dirty stopouts are girls who are one night stands and stay over at guys house’s after a nights out to fuck