euro-truck-simulator-2
I've seen this question, which has no accepted answer, it is implied that the cabin size is only a visual change and doesn't affect the gameplay.
But this is about the chassis. I've seen screenshots of trailers that (from the looks of it) just wouldn't fit on a 4×2 truck. So I'm wondering, if you refit a truck from 4×2 to a 6×2 or 6×4 do you only have the downside of the reduced tank capacity? Or do you gain anything, like the ability to tow larger trailers with more valuable goods?
The truck you're looking for is a fictatious one called Scania Evo. You'll find various versions of it from different sites. Here's one.
As for the wheels you can download tuning mods that will add such wheels to the modding shop, like this one
I did an experiment, but I couldn't find such a short route. Here's what I chose:
It's 511km on land and 370km by ferry, which of course come together as 881km after I attached the trailer:
And here's the job completed screen with the reward details: 
My base reward was €15,813, long distance bonus €1,421, which is 8.9%, in terms of XP: base 516, long distance 129 (25%).
My ranking screen: 
This cargo qualifies for rank 2, because the distance is more than 350km, but less than 850km. I should get 10% extra money and 25% extra XP, which I almost got (8.9% and 25%). If the calculation included the ferry distance, I would've qualified for rank 3, meaning 15% extra money, which I clearly didn't get.
Ferry distance does not count toward the journey distance when calculating your reward. Only on your gps.
Best Answer
Okay, I downloaded a player profile with some play money (1 billion Euro, all skills maxed) and did some testing. I bought the smallest cabin and chassis and took a screenshot of the available jobs. Then I upgraded to the medium sized cabin, took a screenshot again, upgraded to the large cabin, took another screenshot. Same with the chassis.
Afterwards I compared the screenshots. The result: the jobs were always the same. So I guess the switchable parts are really just for show.