Here's a couple ways. You can use or make some plug gauges, which are just dowels turned to fit precisely fit the holes, then measure the distances to the outsides of each pair, subtract the radii, then trig it out.
If you're going to make some precise fitting plugs, you could also make them into transfer punches by turning some points on the ends. With the transfer punches in the holes, hold some sheet or plate stock against them and give it a tap with hammer.
Anyone know the spacing of these holes? Its kinda dificult to measure them.