I want to do the same thing. I tried it but first of all, I have the same extra space problem on the right.
I added
site { overflow-x: hidden !important;}
but it didn't solve the problem.
My second problem is that by adding a div tag into my html, I have to write the content of that "Bio" section also with html. Is there a way to create a customized div like that but still using squarespace tools to add gallery/text and other conent inside?