I ran into this problem on my 124 chassis car. I removed the compressor and manifold hose as an assembly, then drilled the bolt head off. In my case I was pulling both out anyways; it was perhaps a bit more work removing them as an assembly. Can you disconnect and remove the manifold without pulling the radiator - I couldn't on the 124 chassis.
Anyways, the bolt is quite a common item, easily available at a hardware store, Home Depot, etc.