This, if you do not have an air arc or plasma, welding up 2 slots I would think would be the easiest/fastest.If it were mine, I would sink a cutoff wheel flush with the floor, then reweld the square tubing back after doing the spindle repair
Best choice might be an air-arc or plasma if you have access
That's not a bad idea.Is it just square tube? If so I’d lop the end off with the spindle and cut a new piece with a square sleeve inside of it.
Just cut it off as close to the top of bumper as possible and grind it smooth. Then install something OTHER than a trailer spindle and count the old remnants as stiffening. Why put it back to the way it broke before?
https://ck5.com/forums/threads/diy-swing-out-tire-carrier-advice.325469/

You beat me to it, I would use the hole saw.Personally I would use a hole saw to cut through the top and bottom of the bumper to remove the old spindle. I'd then cut two fresh cores out of a piece of plate steel the same thickness and weld the bottom core to the new spindle so you don't have to try and weld inside of the bumper. Then you can chamfer the bumper faces and the two cores, tack weld the two cores to the spindle and the bumper then do a root weld and a many filler welds as needed to bring it up to past flush. Then bust out the grinder and smooth it out.
