Dump/grain truck double frame question

Seems there’s a ton of opinions and then some out there about frame modification. I have a good grain box on a junk truck I’d like to swap over someday. It would be nice to have it on a truck stout enough to handle a full load and maybe an occasional pup. It will hold around 700 bushels so would likely have to gross 60,000+ without the pup. Couple questions I have is
1. I see a lot say double frame typically only needs to extend past rear spring hanger of front axle. But you do see some that go all the way to the front. When or why would you need to go all the way to the front if ever?

2. When it comes to pulling a pup with a straight truck, do you need frame and axle specs of the truck to meet that extra weight legally? Or does that not matter as it doesn’t put weight on the truck sitting still?