here is the legacy 600exl ornamental mill, i know how it mounts, if i need to drill any holes i want to be certain that the router is perfectly centered. the 4 hole mount is for a porter cable router and i have a dewalt with a 3 screw mount.
That is a great looking old Legacy, the forerunner to the 900.
I can't tell from your picture, but have the 'rulers' been glued to the carriage yet? If not, all you have to be is close, and you can glue the rulers to align.
Fortunately, my legacy was pre-drilled for my router. If I was in your situation, I would do the following.
Use your router and circle jig to cut a circle that fits snugly in the mounting plate hole.
Bore the pilot hole you used to cut the circle to 1/2 inch, and put a 1/2 inch straight cutter in your router.
Use the circle to center your router in the hole, and transfer the mounting holes to the plate.
This should have everything in perfect alignment.
Hope it works, it's a great tool to play with.
Dont forget to look at Magnate.net for all of the legacy ornamental mill bits you need. They sell direct from their website and from Ebay.