I'm starting to doubt the case will fit, Apple lists all new cases as 7/8 but I have the Smart Battery Case which Apple does not say is compatible with iPhone 8 and if the case fit wouldn't it be compatible. I notice some older cases are listed as compatible with iPhone 8.
So it could be to tight, I think the iPhone 8 is anywhere from .1mm - .3mm bigger in size depending on what section of phone.