This is the one I am using and it makes a huge difference over the G or the 45mm mikuni.
SCREAMIN’ EAGLE® PRO SUPER BORE
51MM CV CARBURETOR KIT
This is the highest flowing carburetor Screamin’ Eagle
has ever tested. It will satisfy the flow need of the
largest V-twins. This Harley-Davidson® exclusive kit
is complete with carb, intake manifold, higher flow air
cleaner and mounting hardware. This is the largest CV
style carb available and has easy access to main and
pilot jets without removing the bowl. Recommended for
engines 95 cubic inch and larger. Uses stock throttle
cables. The 51mm CV Carburetor is not cruise control
compatible. For race applications only.