Back in the day the easiest way I found to get the hole pattern correct was to use an UF trunnion laid on the rear of the receiver to mark the holes. I'd lay it where it needed to go then hit it with some spray paint to mark the holes. Being that you are drilling an already riveted firearm you'll either have to get a trunnion using the same rivet pattern or just accept losing some accuracy due to the offset. I'd always drill undersize holes and dremmil to fit regardless so I don't think that would be a huge deal either way.
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