Rebuild/recreate lower exit blocks to install lower sheaves in.
These were double halyard exit blocks, machined out of a block of aluminum w/ a face plate welded on.
A single structure which takes the place of both double exit blocks could be made from 3 sheets of 1/8" aluminum placed crosswise to the mast, sticking out of the exit block holes, with the two pairs of sheaves between them. We have the necessary SS rod to act as shafts for the sheaves. The shafts would have holes drilled through each end for cotter pins.
The assembly could be kept in place, crosswise, by either of a couple of methods. 1) use two more sheets of aluminum outside the three, just long enough to have an interference fit with the inside measurement of the mast. 2) Use "angle iron" (aluminum) pieces that are screwed into the mast (at the existing screw holes) and screwed into the assembly pieces.