If it were me, I'd try and put the largest pieces at the bottom. From there stack the smaller pieces and use epoxy to connect them or drill out rocks and use acrylic rod between them to create some overhangs and arches maybe. Here is an example of 2 rocks I connected by drilling and then inserting acrylic rod (the 2 rocks where the top one touches the back of tank)
Create some spaces between the rocks and place some rocks closer to the front glass on one side of tank and then further away on the other. Make the height of the rocks different. Provide places for the fish to hide and swim in and out of. Try to be creative in placing the rock.
Paint the back of the tank black or some other dark color. It will help give some depth.
Stacked rock is best for fish, IMO. It gives them places to hide and fully get away and feel secure. Things that humans find pleasing like caves and arches can make for stressed fish.