So 2 hours later and several scratches from jagged fibreglass shards, I'd replaced all the alloy blocks with 40mm wide X10mm thick and 60mmlong blocks of high-tensile alloy

these are what's left
NB: pic is as they were removed from the hulls as if you were at the stern .... so the top left is the front left and the bottom right is the rear right block.....
The 'larger' block 3rd in the top row is the one for the missing altogether mounting point in the pic in the above post