The easiest version of the dreadnought to convert is the venerable. AoBR guys can be used as well (hey they're cheap on ebay), but they are a bit more difficult to work with. What you're going to do is cut down the arm sockets to symmetrical shapes.You'll have to cut down the pieces a significant amount to get them to this size.
http://www.kjmagnetics.com/ is a good source for magnets.
Cut off the plastic pin on the dreadnought arm and place another magnet. Make sure you test the polarization of the magnets before you glue them.
Now you will need plastic tubing for the barrels.
Using the smaller of the two, determine how long you want your barrels to be. Place the tube in the sponson and mark with a knife where you want it to cut off.
Glue your piece in the sponson. Do this one by one, so you can gauge the length of the barrels off of your previous barrels.
Now all of the holes/cuts from cutting down the sponson need to be covered. Instead of going out and buying plasticard, take the plastic from any package you have lying around.