I can't help with coil selection, however, I wanted to address this suggestion.
I would highly advise NOT T'ing air springs. Putting the "air bags" in one circuit will completely induce body roll, causing potentially dangerous situations- especially while towing.
When cornering- or reacting to someone else's careless driving- the suspension is loaded on the outside tire(suspension/airbag/etc). If the air is in one circuit, it will push the air to the inside tire/bag, inducing body roll.
It's essentially adding a sway bar; not an ANTI-sway bay, a SWAY bar!
I hope that makes sense, and I hope anyone who has T'd their air helpers will seriously consider changing that. I promise handling will improve, and it will also be safer.