A great tool to enlarge holes, is to use a deburring tool. I use them at work, on particularly some really hard surfaces. It really loves mild steel
However, I know how hard Cast is, particularly when you try to cut it with an Oxy Set. Pain in the butt to heat up and cut.
Word of caution, Eye goggles are a must, as fine filaments of steel fly about.
http://www.sheffieldblades.com.au/bu..._deburring.pdf