7,5t trucks transport how long wooden beams?

Hello, I’m covering my roof anew and now I’ve already ordered the beams at the sawmill. Now I have to pick up the beams. I have a 7.5t truck, closed box (no tarpaulin, but solid walls…) behind is such a landing board. Unfortunately, the loading area is only about 4.8 long. My beams are however 6m. Now I just ask myself whether I can just leave the loading board open. Of course, I’ll make a red flag on it. Can I drive like this? Or do I have to use a red flag. r lend a longer truck? Greetings PS: Of course my father drives, am 17…