I've got a 21 racing on mine and its really great. With a smaller pipe (like a boss, 4p) the bike rips all around. Tears off the line and has decent top end, but all that depends on gearing.
I use the spigot with a like 40 degree rubber elbow so I didn't have to cut the fins on my head. The bowl bolt (for jet changing) taps the head and vibrates a little, but its pretty decent.
So yea, I'd go 21, but I doubt there is a huge difference with a 19. If you want the carb straight forward, get the clamp. I personally believe the price difference for the racing version is totally worth it. Easier jet changing, spin-on top, and overall better construction with the metal bowl. Win-Win. Also, if you are running the carb straight forward, the pull choke will be in-accessible with the frame in the way. So yea, get a spigot and run a slight angle.
Notice the angle of the carb with the clamp:
On a maxi frame the angle is even more extreme. Still runs fine, but still quite the angle.