The 500 Jeff is a fine round. It uses a .510 bullet which is more common then the .505 (Gibbs). I have heard that it is difficult to get to feed so I would invest in an experienced smith(ie: someone who has built a few 500 Jeff's before and knows what it takes to get it to feed) to do the work above all. The Jeff has a rebated rim and less powder capacity/shorter neck then the Gibbs. I asked a similar question a while ago. That should give you some good info;

http://forums.nitroexpress.com/showflat.php?Cat=&Board=bigbores&Number=52933&Forum=All_Forums&Words=505%20500%20gibbs&Match=And&Searchpage=0&Limit=25&Old=1year&Main=52774&Search=true#Post52933

Also use the search feature of the site. Should turn up more info for you.