Hopstop suggests that LGA to 5th and 42nd is 57 minutes right now whether you take the Q33 or the Q72 to the 7.
So it sounds like adding the Q72 to the mix is a good idea, and if it's at an hour when the 7 express is running, it might even be worth waiting for the Q72 rather than taking the Q33 if it comes first.
Out of curiosity, if you take the M60, which train do you take? N to 42nd and walk over?