Lee- there may be other methods but I suggest you carefully drill out the rivets, then carefully drill and tap with 10-24 (recommend using a combo drill-tap bit as it makes a one shot procedure), remove the end cap with reefing lines and outhaul attached. Once the cap is off you can pull the blocks out to inspect them, untwist, run new lines using messenger lines to the blocks, etc.
There are other threads on this site about those butterfly or shuttle blocks This thread here http://www.freedomyachts.org/viewtopic. ... cks#p48904 shows a diagram. With the end cap off, you can pull the shuttle blocks right out. You may need to wrangle a bit with the forward lines entering the boom from the mast to allow enough slack to pull out.