TD-
737-300 has a max alt of FL370 - the wing was borrowed from older models and isn't often able to go that high when you are making money. If you are heavy, then 340-360 is kinda your cruising range. In a 737-400 I have maxed out at FL290 when we were on our way to timbuctoo...
Also, cruising level has a lot to do with winds and turb...