Thank you for sending your message. I am finding it hard to verify that Rebecca Taylor was Zachary Taylor's aunt as I go on the internet and cannot find Rebecca Taylor's name listed in zachary Taylor's line.
Anyways - to maybe help you - you can go to rootsweb.com and type in William's name - it may show up in a line for the Taylors' also you can type in William's Taylor's name in the search on the internet as well and see if anything comes up.
By the way - is your last name Freeze? As I have Freese in my ancestry.