Which or who's form are you filling out? I am assuming you are talking about a form that the fortune 500 company gave to you.

If they only deal with Inc's then you will probably need to set up a corp.

Ask them if they will deal with an LLC, then based on the answer you can decide which way to go.

Just make sure the extra burden of a corp is going to be worth the business you would gain from dealing with this company.

Hope that helps.