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There is a chance that the tang is soft enough to cut with a hack saw, I have known this and the spine also. It only needs to be soft enough.
There is only one way to find out.

Joe

Here's another way. Knew it was here somewhere.

There was an old post of Charlie Lewis using blocks of wood to clamp the tang. It was one of his first frame backs. He used an hack saw blade with the kerf removed. The hocks acted as a jig to space...