Couldn't you make in the shop some adjustable shim packs from bolts and nuts, and mount to floor in a grid pattern.
Get them all set level for bottom of stone slab, then lay some rows of stiff concrete...somewhat higher than shims...then set slab and then the laid rows of stiff concrete would spread out and the shims would stop the slab where you want it, let it alone and concrete will setup.
You could plan to hold the laid stiff concrete short of the edges and latter dry pack a colored grout and trowel it to a 45* angle or whatever angle you desire.
After the grout is cured...seal the Bluestone and the angled grout at the same time.
The slab will not fit in the wood opening . I oversized it on purpose . The opening is 63x37" and the slab is 64x40 . I want the stone to be on the wood so no grout or finishing mortar would be used . The slab is rock faced so I wouldn't want to cover any of that up and that's why I chose to do it that way. I think I am going to raise the concrete up until I have about a 1/2" from the wood surface and then apply a thin layer of mortar and while its still soft I will lay the stone!