ORIGINAL: MinnFlyer
It won't be possible to drill new holes so close to the old ones.
If and when you ever need to remove the mount, make a plywood square a little bigger than the engine mount's footprint with through-holes for the screws and place that between the mount and the firewall. That's a much better fix than using washers.
Here is a couple of pictures to show what I am dealing with. The bottom bolts on the mount are very hard to get to.