When you are dealing with ANY wildcat cartridge you have to start with the reamer, and there is NO standard, every gunsmith has his own version, Different neck diameter, different shoulder angles, different throat lengths, ect.
Before you decide on a reamer you should decide on the brass you want to use, whether you want to turn necks or not, the bullet type (VLD or nonVLD) and bullet weight (aprox) that you want to use. All of these things go into determining which reamer is the right one for your rifle. Then there is the availability of Dies, Redding makes a 280 Rem Imp 40* shoulder S type dies, all you would need would be the proper sized bushings and a "custom" set might be unnecessary.
One of the first custom rifles I had was a 257STW. Turned out the neck was a tight neck, so brass had to be turned (which was OK) but it had a LONG throat and would NOT shoot VLD bullets. This was part of my learning curve, I now order my own reamer for every rifle I have built, after I buy brass and decide on bullet class etc. This way there are no surprises AFTER the rifle is built.
Wildcat's are a lot of fun, BUT you need to think the entire process through before you start.