They dont fit perfectly, the GW kits have a little bit wider arm studs, but the FW dread arms come with their own shoulder cowling so with a little filling and glueing it wont hardly show, and if you are using magnets to attach the arms it becomes even less of a problem.
Keep in mind that FW makes arms sized to fit the plastic GW kits as well as their own resin kits, but theyre for Imperial dreads rather than chaos so they just wont have any chaos iconography or gilding on them.
Good luck with the conversion!