After using the spot welder (on my AMD-63 build) as I had modified it the original lower tong tip came unsoldered (I used low temp solder...bad). I was thinking about using a higher temp solder when I remembered reading that someone (??) used a MIG tip. So I bought a package (about $4.50 for the .030" hole) and updated my lower tong with the new tip. Since the MIG tip has a threaded end I used a nut to hold it in place. I did have to chuck the tip in my drill press to remove some material (I used a file while the press was running on the non-threaded end) so that the both tips closed on the lower rails correctly.
The test results were very good. The picture of the spot welds doesn't do justice for the job. The welds are quite smooth. Also it seems like the little hole in the MIG tip doesn't cause any problems. I give the MIG tip idea a thumbs-up!!