Ron Covell has some metal working videos that have taught me a ton. I recommend them highly.
http://www.covell.biz/He drills out the smaller holes with a step drill to get them perfectly round. Using a Whitney punch he makes a perfect slug to fill the hole. Then holding a dolly behind the hole and with the slug in place a few light taps with a hammer expands the slug slightly to hold it in place for a perfectly crafted tig weld. His work is impressive.
Best of luck.
