Scoping a Mosin Nagant 91/30

My neighbor has a mint 91/30 that he wants to scope. He bought the original scope and mount to put on it. I know the scope/mount has to be concentric with the bore but barrel is tapered so indexing the receiver by chucking it up in my lathe is out of the question. Any ideas from someone who has done it. Is there any jig, template, schematic, etc... that would help. It is a round receiver and not the octagonal one. When mounted it looks like there would be two index pins and two screws to hold the mount in place.