How much exposed 2x3 tube is there between the end and the first thing welded to it? What tools are available to make the cut(s)? Is lengthening the tongue desired, optional, or not wanted? Do you have the ability to drill multiple ~1" holes?
Let's say that there is 18" of exposed 2x3 tube. What I would do is to cut the bottom 1" off the tube for nearly that full 18" length.
Buy a 2 foot long receiver socket tube and insert each of the likely couplers in it one at a time. Measure from the opposite end down inside the tube to the insert, find the longest one. Subtract 1/4"-1/2" from the longest insert's measurement for operating clearance. This is your cut length.
Mark this length also on the top and sides of the receiver tube and lay out and cut/drill several ~1" holes in that length on each of the 3 sides. I would not align them, offset them.
Make the cut on the trailer tongue and then slide the receiver tube over it. Weld the receiver tube to the tongue tube thru the holes. I would not weld around the end of the receiver tube as that has the potential to do more damage than good by creating a stress riser all the way around the tongue tube (effectively says "snap here please").
Take a piece of 1/8" or 3/16" thick strap (no need to be thicker than the tongue tube's wall thickness) say 12"-18" long and 1-1/2" wide and miter the ends. They don't need to come to a point, but you definitely don't want a full width square end. Form it to be 1/2 on the bottom of the original tongue tube and 1/2 on the bottom of the receiver tube. Weld it into place. You're looking at a step of about 1/2" so taking a pie cut out of the bottom of the tongue tube to make the transition angled instead of abrupt would be a very good idea.
