The Finish MNs were made with .3085-.309" groove diameters, so that's what you'd be looking for. But “Fins” in 95% to 99% condition cost about $500 to $600.
However another option which is quite good is to get any MN you can find in good shape, but with a shot out or rusty bore, and have it rebarreled.
I have done 5 of them now and they come out great.
You can get MNs for about $100 with bad bores and you'd need to add about $200 for a rebarrel job.
A turned down bolt and a scope mount will cost about an additional $125 for both.
So all that added up is still less than a good Finish MN before you get the bolt and mount added. So for making a great shooter (but with no collectors value in mind) doing a build “from the action up” is the best option.