To make it simple : Empty containers can overhang. 38' bottom deck, 2' overhang (EMPTY). Loaded containers must "have support" under all 4 corners. Pretty much, if you have a loaded can it's got to be over the "plane" of the deck.
Back in the day though, I've seen Cal. guys with a 250" wb coe put 3, 20 footers (empty) on a 45' trailer. Crazy overhang front and back.
Also, the guys with 53' steps dance a thin line with closed tandem and spread in Ca. with weight concerns on the trailer .
It's a mixed bag really. I get a lot of loads that I wouldn't get with a flat. And I miss a lot of loads because they won't load a step. Since you can't have one of each type of trailer in your side box to pull out when you need it you're going to just have to understand that is the way it is. Choose your poison.