Unfortunately, it sounds like it's time to pull the starter and bench check it. See if you can turn it by hand. Could be it just gave up the ghost! Most people leave the 6V starter installed when they convert to 12V and over time it may have just failed. Don't know what the cost would be to rebuild for 12V.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm1221.htm
