if your slab is the normal 1-1.25 " thick granite i really doubt those factory hinges will support the weight.
have you considerd the effort of just lifting that kind of weight each time you need to get in there to do something?
may I suggest instead of real slab granite try looking for the 1/8" or 3/16" thick cut 12"x12" ...or larger tile that many home improvement stores sell.
or you could use marble tile.
i had considerd using the granite tile as a counter top myself.
much lighter, cheaper; still real granite and the joints use clear epoxy so they fit together clean.
good luck with your project.