In my limited experience , and i only tried this once ! The square GpA Tarmac beam ( red ) and steel modded rear arms mean problems with driveshafts, no-one i know runs this configuration , although loads of people use a std round rear beam and mag rear arms , though you should be aware of this ! Also square beams bolt up in a completely different way , the eye bushes are designed to be solid and there is no top mount on the diffs , this means GpN or GpA diff mounts are essential .
My tuppence worth anyway !
cheers