Ed actually meant that the 40mm guns had been replaced by 3"/50 cal
guns on St Paul & Helena during the 1950s. This gun is
available in the Skywave US Weapons set E-6, although you only get four
in a set.
So the question arises - how many 3"/50 mounts will you need? As you can see from the link posted above -
st paul link
http://www.history.navy.mil/photos/sh-usn/usnsh-s/ca73.htm
and
http://www.navsource.org/archives/04/04073.htm
her details changed a bunch over the years.
Per "Cruisers of the US Navy, 1922-1962", by Stefan Terzibaschitsch,
she had 10 3" guns (five twin mounts). (assuming that this wasn't
a typo for "still had ten 5" guns". I'm still looking around to
confirm this. Stefan T. and his printers frequently let the facts
get out of adjustment.)
Like Ed said, you'll have to delete all the 20mm guns the kit provides,
which isn't much of a problem except at the bow, where the platforms
are built up for the 20mms. That will take some slicing and
shaving.
The superstructure had grown a bit from the 1943 look in the kit to the
1964 look, and you can get a good sense of it with the series of photos
while going alongside USS Boston in 1968.
And yes, I'm just in the planning stages for a mid-1960's St. Paul model too.
Best,
Rick