You might look at what the "receiver tube" is blocking before you weld it. If the frame is welded and the tube is loose then there may be a reason for having a pin so it's removable for repairs or replacement of what ever it might be blocking.
You might try getting a large bolt the size of the pin hole and tighten the hell out of it and see if that reduces or gets rid of the slop and or, like has been mentioned also try to get a few pieces of sheet metal wrapped around the receiver tube and see if that helps too. Take the tube out, wrap the sheet metal around it, drill or punch a hole for the pin then reassemble. I picked up a small roll years ago at one of the hardware stores, Lowes or HomeDepot, etc.