Bass Problems

Hey
I have a 5 string jazz bass copy, which was set up with with low B strings but factory tuned to high C
Recently i got a real fender jazz 5 string, factory tuned and set up with low B.I wanted to bring the tuning up to high C but couldnt, too tight and too much pressure on the neck.Even when i bought a new set of strings the same thing happened

Then the C broke on my copy, after a long time in use.I tried to replace it with a G from the new set, as the C was actually a G due to the factory tuning.But no, this couldnt take the striain to get to the right octave, even thought the previous G had.

Can anyone tell me why all that happened and what i can do to fix it?
Thanks


So