The bolts on my '93 960 were also quite tight even though it had been
about one year since they were last removed (Girling calipers, I
think). I have to assume that you are also replacing the pads. WD40 is
not going to do it. You need to get a penetrant oil (like "Liquid
Wrench") and apply it about every ten or fifteen minutes for a couple
of hours. Then take a hammer and tappity tap tap like playing drums on
the bolt head about every half hour during this process. The
vibrations will help break up the rust and corrosion. Then try
applying a slowly increasing torque to the bolt. Also try tightening
it JUST A BIT and then loosening, and repeating that over and over.
Then start the process over again.

When putting it back together, clean the threads of the caliper and
clean (or replace) the bolts, and use some quality anti-seize on the
threads. Of course, it goes without saying, that anti-seize on the
pads or rotors is counterproductive.
from Randy & Valerie
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      \__/olvo
 1993 960 Estate