[Wbulk]

What is the section width of the tires you are mounting on the rims. I have 10" rims with 5 1/8-5 1/4 Back spacing and 295s. My section width (bulge) of the tire is 12 1/2", and is just about flush with the fender lip. I reached behind and felt the gap in the area you are talking about a few weeks ago and it felt like there was maybe and inch left tops. So it looks like that would be very close.
One other issue is it appears all of the cockpit fiberglass tubs may not be installed exactly the same. If it was pushed back a little you will have less room. So with your 6 1/4" BS the section width of the tire would be about one inch in from the fender lip. But it's really going to be close to that panel on the inside. A wheel spacer may help. Another option would be to modify that panel like Larry did. If you want me to measure that point exactly I can tomorrw, advise. But again our cockpit tubs may not be installed the same.

Wayne