John has given you some good advice, in my opinion.

When the corrosion is this bad you have to take the possibility of
internal rusting into account.

I've a hunch the cluster is suffering from a quickie repair made in
the distant past, which would cause me to distrust the other welds
associated with that cluster.

Take special pains with your jigging before doing any cutting. The
usual procedure is to create a tack-welded secondary structure with
good triangulation so that cutting away the rotten stuff will not
disturb the alignment. Another method (for those who know how) is
bondo, bailing wire and wooden struts. A third is to affix the
structure to the hangar floor. The need for special care at this
stage is because the welding needed to do the repair will introduce
some distortion. Establish good trammel points before doing any
cutting.

I've found that when the corrosion is this bad the MORE of the
existing structure you cut away, the LESS chance of ending up with a
'crank' structure.

-R.S.Hoover