I'm a welder by trade, my vote is for welding the sleeve.
The problem with bolting stuff together on a hollow box (like the frame) is is that you almost have to have a spacer inside the box at each bolt to keeping it from crushing the box frame (like it looks like your shackle bolts are doing). The advantage with adding the extra brackets to the frame is that it spreads out the load to a wider footprint. Bolting the extra plates on was just a way around welding them in place.

That said, after the pipe is welded into place, adding a 2nd plate to the inside and the outside, then welding the outside of those plates (I would weld all the way around the extra plate) would add a lot of extra strength. Your cutting a pretty big hole in the frame, spreading the load a bit more wouldn't hurt. Gene