you will be completely fine with 2x4's they are more than enough for a 180. just be sure you put it together right so that the weight is distributed evenly.
this is how I did mine so that the upper and lower frames were connected and then supported using 2x4's you will have a far more sturdy stand this way than you will by just screwing the 4x4's into the frames
this is how I supported evenly(I haven't added all the 2x4's legs as in the previous pic yet) my tank is a 220 and it is ROCK solid
good luck