Actually, you do not have 180 days. You would be illegal the day after your OPT expires unless you are approved for another visa. Also, I do not think that you would able to adjust your visa to a H-1B if you are illegal, but you might want to double check that with someone else.
Let me caution you that overstaying your visa is not a prudent move, especially during these times. BCIS has gotten more strict and a lot less forgiving post 9/11. Even if the 3/10 ban does not apply to you, you would most probably run into issues when applying for another visa if you have been illegal before.