Julian,
I did a rear disc brake conversion on the MGB using the Wilwood spot calipers:
http://www.rc-tech.net/MGB/em/emn.htm
It doesn't take much hand brake I doubt you will have any trouble hooking it to the existing handle.
If you look at how I did it, it's ok but not ideal. The caliper has a groove in it. Since the caliper squezes from one side, the entire caliper needs to move a little. I used rubber to make up the difference. Works ok on the MGB. If I had it to do all over, that is really what the slot on top and bottom of the caliper is for. I think mose mount it so the caliper slides.
Gary
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