soaking them wont hurt....I think you will find that the manifold is
warped and this will at least make it very hard to reinstall(the holes
wont line up with the head holes). Give it a try but look into a new
manifold too. If its available it wont be very expensive and well worth
it when its reinstalled. There is also the chance that your old one is
cracked,  so the only fix is a new one.