Not that there is anything wrong with placing a heavy rock in the aquarium, but i can see where your fear stems from...
I've always personally placed rocks ontop of the substrate, although i know to some this is a big no no... this way at least in my mind if it does slip, its not as likely to just crack the bottom of my tank, and 2nd i feel more comfortable knowing its not touching the bottom directly so if it does tilt and fall from fish knocking it over, it won't crack as well...
But with the sort of weight your talking about, i would also be hesitant to place it into a tank unless i was 100% sure i could put it in and take it out with ease and no worry about dropping it...