I feel this is would be the easiest solution with just a one word code change. I had implemented this quite sometime back and may be I did not even realise then someone could use this to change the blog layout.
Yes, it will get overwritten by updates. I tried to avoid this using CSS but results were difficult to achieve due to presence of a extra div .clear element. Cannot use css http://www.w3schools.com/cssref/sel_nth-child.asp to target the 4th child after changing the width of the .sixcol to something like 22%. If you hide the clear element(display:none), then your layout can break if the box sizes are not even. Hope this makes sense.