Thank you for this information, one main thing to note for anybody coming across this post in the future is I had to modify the stored process itself in the management tool to disable the option to include code for stored process macros, I believe the stored process was defaulting to HTML and ignoring the headers setting the content to .xlsx format.

Additionally for anybody who might be coming across this, the formatting for the URL that I was struggling with was &_username=[Username]&_password=[Encrypted Password] at the end, I was struggling with finding the formatting for a short time. 

Thanks!