One thing I have found (the hard way) working with guard rail is that life is a whole lot simpler if your post spacing lines up with the odd bolt hole spacing on std 6'-3" and 12'-6" guard rail. 

Be careful even blowing a torch hole thru heavily zinc galvanized guardrail. That zinc is extremely hazardous to either weld or cut with heat. don't breathe the fumes. A lot better to get the posts at right spacing and not have to cut or weld the rail at all. Hole slots allow for a bit off but best to make a couple 2x4 spacers with bolts at 6-3 or 12-6 to get/hold posts in place while concrete sets.

Jim at Dawn