I know how these projects go.
Start out with one thing and before you know it you are rebuilding the whole truck. This is just like an old house; Go to fix a leaky faucet and before you know it you got the bathroom torn down to the studs.
Reading your post and thinking what I would do with this truck;
I would remove the bed. get four to six dudes (or my ex girlfriend) and lift the box off and put it on some horses. I don't know if it is necessary to remove the bumper and hitch.
Considering the rust on this truck I would consider that a Wisconsin fire wrench would be required as bolts may need to be heated up or cut off.
Possibly remove tank if necessary. Probably a must, probably other needed to be removed to make way.
Sand blast the area of the broken mount, maybe even sand blast other areas as needed, required for inspection on metal to make sure it is solid.
I can't weld for ####, so I would pay somebody to weld this for me, Like a body or frame shop or find a
@kennythewelder dude to weld this for me.
After welding, clean everything up and paint with POR-15
Replace bed ect..
Drink Beer and tell friends..."Whose the Man Now"