Just revisiting this thread having decided to go for the removable bracket option. Taking out the foam but was an quick easy job and I’ll take Swiebertje’s advice of putting something different back in.
Next question is what to do for a backing pad. There’s what I assume to be a ply pad laminated into the transom which extends out about 300mm from the centre line. At the recommended 400mm I would be going only through the grp so obviously some substantial backing pad is required. What thickness ply is recommended? Presumably the bigger the pad that can be fitted in the better? Is it advisable to glass/epoxy it to the inside of the transom or is marine sealant sufficient?
Any help much appreciated.