how about a pipe inserted into the bore of the valve if you have room above it; Might also work inserted from outside.

5200 can also be melted, heating the bolts from the inside will probably allow them to be unscrewed or pushed out.

Once the bolts are out you can also cut the 5200 with a thin flexible wire (piano wire); Wrap the ends of the wire around some dowels for handles and use it to saw the 5200, once the wire heats up it will cut fairly quickly.