Hey bud in regards to the twin 12" or single 16" question. The twin 12's will cover more surface area if you do the maths. Twin 12's cover 226.08 square inches, whilst the single 16 covers 200.96 square inches.
Running with this. If you can fit a single 18" it covers 254.34 square inches. Which would be the better of the 3.
Personally if you can fit the single 18" i'd be going with that. Simplicity is always better Hope this helps