The SUs are exactly the same size as the Zenith Strombergs. I haven't noticed
any difference in power, maybe more with the SUs since they are easier to tune.

The HIF6s were used on B20s starting in about 1969 and have a few advantages.
They have the float chamber in the bottom like the Z-S so you don't need an
angled float chamber for them, and they also have a bimetal device on the jet
for limited temperature compensation. They have seals on the throttle shaft
so they don't need rebushed as often (if ever). Since they don't have the
jet hose they don't have problems with leaks there either. The mixture is
adjusted with a screw that moves the jet up and down. The screw is located on
the corner of the float chamber.

I believe either SUs or Z-S carbs can give better performance than the
Weber kit if properly tuned, although the difference is small.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma