Only issue with MIG welding the holes shut is the welded area is going to be harder than the steel used. You will have great difficultly drilling and tapping new holes without breaking a drill bit or tap. Yes I have done TIG welding and IF I did that I would use stainless rod which would be tougher than carbon steel rod but flows a lot faster and easier. With TIG the metal would have to be absolutely clean.

So this machine is a 2x4 foot and was done prior to the Saturn 2. Can you post a picture of your machine?