The width of the brackets on the side mounted to the original factory brackets is about 2 3/16". The holes for the M5 screws are drilled as necessary to give the 13" spacing. I attached aluminum 3/16" thick pads to the to the brackets at the holes to provide a little bit more thickness and strength to the ears of the bracket. The pads were attached using JB Weld and clamped until cured before drilling.
The angled side of the homemade bracket is 20 degrees and the angled plate is 5" long. Three holes for M6 screws were drilled along the angled edge. Three holes seems to be plenty. I just did a 1000 mile Iron Butt ride with absolutely no windshield problems.
Once you get the brackets mounted in place on the factory brackets, assemble the windshield with the cut brackets. Place the assembled windshield on your new brackets and clamp it in place. Using your white paint pen, mark the windshield brackets through the homemade bracket holes. Disassemble the windshield and drill out the holes you marked. There should be at least 6 of them (3 on each side).
Assemble the whole works and see how it fits. If you like it, take everything apart and paint it.
When the paint's dry, put it all back together and enjoy!
A few other notes...
I used a foam can koozie to provide about .5" of space around the headlight housing when fitting up the windshield. That will leave enough space so that nothing rattles later.
You will be doing A LOT of assembling and disassembling. Measure twice (or more) and cut once.
I have a pretty decent arsenal of tools. I have a drill press, band saw, belt grinder, bench grinder, etc. These tools make the job much easier than using all hand tools. BTW, you need a bunch of hand tools too!
I made some assumptions in this description that the reader has some skills and can make some determinations from the pictures.
Here's a couple more pictures that may be of some value.
