Improving posture behind the wheel

I m 6'7" and can lay flat and not touch either wall in a Freightliner or International. In a Volvo 780, my feet are up against the cubby hole on the passenger side of the sleeper behind the passenger side work station

 

I am 6 feet tall and I had some of the same issues. It does take a while to get used to driving long distances. But remember, if you don't stop every once in a while and get out of the truck, you could develop leg/back issues. My rule was to get out of the drivers seat every two hours. Even if it was only to get something from the bunk. Raising one hand above you and grabbing an overhead compartment and lifting yourself an inch or two can help. Heck, just having my arm up like that felt good.

 

You'll get use to driving long distances the more you drive..
It's a new position for you and your body isn't accustomed to it.

Time, my friend.

 

put the cruise on, and rock your feet, from heel to toe. Do this about 20-40 times to get some blood moving in the lower extemites. Tighten your calf, then relax, do this 10-15 times, thighs next.... then butt.
I like to hang an arm to the side of the seat of a mile, then switch, and then hold an arm over your head for a mile, then switch again.
The name of the game is to move, even just a little like the seat height, or lumbar.
They say to put the seat back at 110degrees, knees above you hips, and close to the wheel as possible with your chin tucked in to your chest.
I use what ever is comfy.