Back when ...
I ran a round-slide 38mm Mikuni for awhile on my '46 FL 74". It performed well, but with fatbob tanks it was neccessary to run a throttle cable with a 90 degree elbow where the cable entered the top of the carb. This would occasionally hang-up the throttle cable. I could usually snap the throttle open and shut to break it loose and get it to idle down, but once, in heavy San Antonio traffic, it hung in the full open position when I did this, resulting in a bent pushrod before I could shut the racing engine down. I replaced the Mikuni with a 38mm butterfly-throttle Keihin from the take-off pile and it worked well for years.
My 46EL has fatbob tanks and as Randal states one needs to run the 90deg elbow to clear the fatbob tanks, however, Randal's experience with the throttle cable hanging up has not been my experience. I've had the Mikuni on the knuck since '78 or '79.