Don't worry about the taper so much. You should be able to move it back that far. Other option, shortening the barrel and breechplug, which will change the relationship of the fence to the barrel, causes the same concern over fit. If when you are finished you have what you think is a noticeable gap, wet the inlet till it is soaking wet, wax the barrel, put it in place and clamp the stock tight from the sides overnight.