Hangers & shackles. Get the ones for a reg. cab. Although I can't see there being much, if any difference.
Easier to do with the box off.
That driver side hanger can be a bee-atch because of the fuel tank in the way & the box on.
7 of us in the shop & none of us could get our mitts in there. Had to get the office girl with her tiny little hands to help. Even then, we glued the nuts to her fingers because she couldn't get in there while holding the nut with 2 fingers.
After all is installed, check the pinion angle. More than likely it'll have to be corrected with some wedges.
And then...(and THEN!) the centering pin head in the spring pack will have to be extended in order to reach through the wedge & into the hole in the spring perch.
GM lists 4 center pin lengths for 400's, depending on the spring pack configuration.
Get the next longest listed. More than likely, your truck uses the shortest one.
Then get an appropriate nut & crank it down to the head of the new center pin with a bit of red loc-tite. Take that to a bench grinder & round off the nut down to the circumference of the pin's head.
Doing this requires that the U-bolts be undone. Do not re-use them. They're torque-to-yield. One time use, only. New ones are something 4 or 5 bucks each.
Inspect the bottom plates that the U-bolts go through, too. Notorious for rusting out.