cheap washer or hardened washer? few people think of that.
if you have to go washer route choose the smaller size and drill thru 8mm.
If all I had was minimal shops skills I would pass on the dinky washer and find some 10 or 12 ga steel. drill thru 8mm so your OE bolt will be tight.
have the bolt in place when you tack weld. eyeball and you will not be concentric.