Can someone help me out and confirm that the horizontal stabilizer /
elevator of an RV-8 at it widest point is just under 9 feet?

The best folks to help you out are at Van's Aircraft, either by phone or
email. From their web site:
"503.678.6545

Monday thru Friday technical support hours are between:

7:00-9:00 AM and 3:30-4:30 PM Pacific Time.

When calling, please have your questions thought out and any applicable
plan numbers readily available. This will help us support as many people
as possible and keep your phone bill at a minimum."

Also, will work.

Ed Wischmeyer