I'm going to guess that you will neck size only once you have fireformed cases for your chambers. Rather than a full blown custom die, I'd suggest you find a machinist who will chuck up a 7/8-14 bolt and bore a section of the needed taper into it. This will shrink your cases sufficient for fire forming. It should be a good bit cheaper than a custom die. If you are really lucky, you might find someone with a chamber reamer that has the correct taper - that makes this a very easy job.