When doing anything with Excel regarding formatting or anything like that your best bet, in this case, would be to let your app generate the spreadsheet. Once you have your data in the spreadsheet open it in Excel - go to the developer toolbar - start recording a marco - make your changes - stop the macro - take a look at the generated vba code and convert that into c# and add that code to the end of your function that creates the spreadsheet. This is how i have always done my formatting and whatnot in Excel. Just keep in mind that the vba that is created will reference a few objects and enums, check the msdn they have the definitions for them there.
Also keep in mind that those marcos basically run inside a