That's definitely a Japanese maple there. They are not as good for root over rock because the roots don't thicken up and grab the rock like the tridents do. JM also need more pruning skills to keep a good shape and ramification longer term.
Having said that you can still do root over rock if you want.
The tree is a few years old so roots are probably already thick and stiff. Maple roots are quite brittle so they usually break if you try to bend them. You can only

and hope that some of the existing roots will fit over your rock.
The rock has no real outstanding features so you can put it any way. It might pay to wait until you see the roots before making a final decision because the roots might fit one direction better than another.
If you are really committed to getting this tree on this rock and the existing roots don't fit you can prune the roots quite hard so it will grow new roots that you can mould to the rock better. That will take another year. I would probably cut the roots until the roots that are left fit on the rock then plant both tree and rock into a large container for 12 months. Next winter check to see how the roots have grown and rearrange new roots to best places on the rock.