Heaviest I've personally towed was another crew cab long bed 400 dually with a 6.5 on a 22 foot car trailer, but I don't know what that weighed exactly. I've seen others tow 2 and 3 car gooseneck trailers, but I can't speak to their frame mods.
Can you elaborate on the idea of the notch being a hinge point? The idea that a notch weakens the frame is common, but I'm not sure how accurate it is. Seems to me that if you bolt and weld a notch in place it would be stronger than the rest of the frame, and less likely to fail.
Its all about loading and how how that load transfers through a structure.
Imagine if you had a 14 foot piece of 4x4 square steel tube.
We could support it on both ends while ten of us stood on it and it might deflect a 1/4 inch maybe.
Because we are applying a concentrated load but the structure transfers that load to the ends of the tube.
The top of that tube we are standing on tries to squeeze itself together.
The bottom of that tube is trying to pull apart.
The sidewall of that tube transfers that load away from the failure point and keeps that tube from failing because of its form
Think about a 1/4 thick piece of 4 inch flatbar that is 14 feet long.
If we lay it flat on some blocks.
I can sit my ass on it and bend it.
I can step on it and bend it.
Take that same flatbar and restrain it in a verticle position so it cant deflect and your whole family can stand on it and wont deflect.
A C notch changes the load from being mostly a linear load to a load that transfers through a hard welded corner or change of angle.
And then all of that linear characteristic and strengh and ability to tranfer a load is done.
Metal structes do not carry their properties through welds.
And the weld is the weak point.
And the heat affected zone adjacent to the weld.
Box the frame way past the C notch and weld it up.
Its all about getting the loads transfered away from your notch.
Not about keeping them on and loading the notch.
You want that "energy" and "work" to have a place to transfer down the frame so it doesnt point load at the notch or the welds.
Plate it over the c notch and past it fore and aft and weld it.
Weld the frame box to the notch.
Then figure out how to weld that new frame to the truck.