Illinois wasn't recording births when she was born so you can't look it up on a birth record.
I found online an index of their marriage but her surname is spelled incorrectly as Copland. You can contact the Massac County Clerk & Recorder of Deeds and request a copy of their actual marriage document. They have marriage records starting in 1843.
The Illinois regional Archive Depository has marriage records 1854-1923.
Another option is to contact the Massac County Genealogical Society and ask if they have copies of actual marriage documents.
I don't think Kansas was using death records in 1874, therefore, that source is out.
I'd enter the into in your genealogy software in the AKA (Also Known As) field with the source. It's not uncommon to names to be incorrect or spelled differently when you're dealing with records that old. Successful Searching!