bagging my dually. Need help!!!

I have a 88 ext cab Chevy dually I'm building to haul my boat. Need help with what to use to build the step notch so the rear of the frame won't bend with the weight of the boat. Its a 22' scarab that fully fueled and lake ready is around 4k lbs. Any info is greatly appreciated.
PS: the truck was a cab and frame when I bought it. Will post pics as the progress continues.

3"X5" 1/4" wall box tubing is what I suggest. It is the same dimensions as your frame and is actually stronger than the stock C channel. As long as you fish plate it, box the rear of the frame, and do the welds correctly you should be good to go.

Please explain what a fish plate is.

A fish plate is a surface plate made to strengthen a joint in a high stress environment. See the picture below of my notches. I have fish plates on all the main joints. Yes a good weld would work just fine, but I did not want to leave anything to chance when I was hauling something.

Hope that helps! Show us some pics of your ride!

Thanks. I can say I now know what a fish plate is. I will for sure be doing those. I waited too long to get that boat to lose it because of a problem I could have prevented.
I've tried to upload pics but it says the file is too big. Trying from my phone.

I have a mostly bolt on four link kit for one of those trucks it is a super heavy hauler if you are interested
it is new and never installed I would take $2500 for it
Let me know if there is any interest.