Wind two nuts onto each stud, and 'lock' them together. Then using a ring spanner and hammer, attempt to unwind the innermost nut using sharp and abrupt hits to the end of the spanner with the hammer.
Failimng that, you can heat the manifold with a blowtorch at the back of the stud, then spray something aerosol delivered (deodorant works well but it's flammable so be careful), onto the stud to cool it rapidly, then try above method?