If you do a couple of things to them you will save them from cracking..
cut each tube where they are tied together and drill one of the 2 holes
on each tube a little bigger(doesnt matter what size hole).. that way they
can grow with the heat( they will expand so they need to slide a bit)
or they worp .. looks good.. I built a ton of headers at Chrysler
EDIT
this is a set I built for my W-9.. totally equal on all tubes