After searching all over the place for exact information on these subjects I decided it was time to provide it all in one place.
Bike is a STANDARD (no bags) with PROGRESSIVE 11 1/2" shocks on it. For reference only.
I pulled the tire for a tire change. Noticed the right side nut had been rubbing the tire. Rarely ride two up.
Tire being removed is a Toyo Proxes 205/55-16
Tire being installed is a Vredestein Sportrac 3 in 205/60-16 The new tire (uninflated) is around 8.5" at its widest part. A little wider and a little taller than the Toyo. Had to make the above changes. Yes, I know the width of the tire will change when inflated. Just stating the facts, man!
Carriage bolts used were SS 5/16x18 x 2 1/2" (Valkyrie Standard) No bags. Quantity 2
Chrome 5/16" flat washers. Quantity 2
Chrome 5/16x18 Acorn nuts. Quantity 2
3/8" SS flat washers for fender rail spacing. Quantity 4. Only used 2 on right side.
Nut cages had been removed already prior to photos.
This is the OEM measurement taken after nut cages were removed. Approx. 8 3/8" from the inside of the bolt to the inside of the opposite bolt.
Carriage bolts only. Measurements were approx. 8 11/16" bolt to bolt. 
Added two SS 3/8" washers between the frame and the right side fender rail at location shown to space the fender out on right side only. Nothing was added to the left side as I was only rubbing on the right. Just slide the washers up there and hold them in place with the bolt. You may need to add washers on both sides. Your choice.

After fender washer modification/spacing. Measurements came out to approx. 8 7/8".
The 2 1/2"' SS carriage bolts required no modification or cutting to install the washer/acorn nut. Plenty of thread and engagement without a mechanical bind in the nut.