I struggled with the same thing on my build, and finally ran across this option, and they have several other versions also. Sorry about the pictures, but I wanted to give you the full perspective on these mirrors.. They are sold by Vision Hot Rod Concepts
www.visionhotrod.com , and they are true fit mirror.. You literally cut and sand the base of the mirror to your preference, and then drill the mount holes you want, and then add the supplied threaded inserts.. I took quite a bit off of the base, because they leave them extra long for you to work with, so more could come off for a tighter fit if you wanted.. These mirrors were about 250 dollars for the set, and that is with the LED Turn Signal option, so I thought they were a bargain.