If you want to add another block, whether it be single or double, you have to utilize one of the holes in the mast plate bolted to the deck (and I saw one available hole in one of your pictures). If you can find the piece that fits into that hole, it has to be inserted from the bottom, which means you have to remove the mast and then unbolt and lift the mast base to do so, which is a lot of work for one block. So, if I get your question right, you are thinking of replacing a single block with a double to do the job. Schaefer made some swivel blocks that accommodated a shackle which could be removed and then a clevis pin be inserted in the hole of the block swivel part which also went through the pin from the mast plate. You'd just have to see if you have the clearance to do so.
