I can tell you that a tline will not be anymore efficient in that space than a sealed enclosure due to loading requirements of a tline. But definitely stick with a single driver for more enclosure room. Best bet, if possible, is to utilize a center console idea or something that will flush the floorboard with only about 5-6 inches up with a downfire idea for a single 10 or dual 8s. Limiting your potential to that space alone will cause a lot of work and investigating to make sure you get something worth the purchase.
Also, try to utilize a shallow mount design regardless of the difference in power transfer and xmax parameters, because those will be less sensitive in a truck space anyhow to the idea you want to utilize. So, get a shallow driver with around a 10mm xmax and one that allows for a smooth response in a small compression chamber, and load the hell out of it with a port, if used, to make up for low end sensitivity loss. Usually, anything that has an ebp of around 47 will do wonders in that environment, But others if calculated can get surprising results.
Best to have someone model some ideas up with different drivers to see what will work.
P.s. I'm here if you need me so pm me or email me anytime. I offer custom designs and may be able to help get one that will work in that space or offer other options when available.
