Hi BB
I used a smaller drift on all the heads (very hard to hold in the vice as it doesn't have any parallel sides), this loosened one screw.
This seems to me, to be a telling point in favour of handing this job over to a specialist, in most cases.
For someone who is regularly doing these, it's worth their while to build a proper jig to secure and support the soleplate during screw removal.
Support is the key here - unbalanced forces, particularly impacts, cause cast iron to fracture. It's strong in compression, but brittle.
