I wouldn't recommend a Sequence Dart in such a small sump. The pump moved 3000 to 3600gph and that means water will be careening through your sump. You'll never stop the microbubbles, especially in a return section that is 8" wide.
For a 180g tank, a Mag 12 or Mag 18 with 1.5" plumbing is sufficient. Or a Panworld if you prefer external. Try to keep it around 1000 to 1200gph worth of flow.
If you have the space, I would put the Calcium reactor in the return section mainly because if it leaks, water stays in the system and off the floor.
I would stick a longer sump in there, mainly for travel time. You've got the space under a 6' tank. Use it.
