concrete slab floor drains install

I'm installing two floor drains in my 32 x 44' thickened edge slab. Slab is still not poured and I still need to add 5-6" of stone inside my forms before I'm ready to pour. Should I wait until stone is in place before I install drains or try and add them first?

I know I'd have to dig part of a trench in the stone but stone trench would also help hold everything in place.