Spare wheel clamp to boot floor

Hi list,

I recently bought a secondhand 'spare wheel clamp to boot floor' for my wire 
wheel 69 and found it wasn't long enough.  I figure this is because of the 
non-standard tyre fitted  to the rim (175/70).  Does anyone know if the wire 
wheel clamp for the MGB is:

             A: longer and if so by how much?

             B: the same thread.

Thanks

Daniel1312

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