I've got a plan for a wildcat i am going to do over the summer. I'm planning on makeing a .204 WSSM. I'm going to use .223 WSSM brass. My question is, how do you make the neck the right size for a .204 diameter bullet? And after that, will the gunsmith be able to rebarell an old 30/06 barrel to this caliber?

Help is greatly appreciated!

Should be a piece of cake. Any old gunsmith should
be able to take care of that.

Well, first off, that's a pretty long action for such a tiny cartridge. I'd look for a shorter action first. If this 'cat does what you want, you'll probably wish you'd built it right the first time. Feeding may be a bit clumsy from such a long journey way beck in the magazine to the chamber. But finding a M700 in 308 or 243 or 260 shouldn't be hard.
How you size brass is with a sizing die. The sizing die can be cut by most any die maker just on case specs alone, but the right way is to design the cartridge, give it to your smith, who'll have a reamer made for the chamber. (These are always bigger than the cartridge, so it won't be the same dimensions.) There's a few things to know ahead of time, if possible, like what sort of throat you want (allowing for your favorite bullet seated to the base of the neck, or longest possible COL, for example), throat angle, freebore diameter and length, as well as how much clearance in the neck and overall. I think, e.g., SAAMI has .008" clearance on the case body. You may want less than that, maybe much less if you like accuracy over feeding ease.
So, you get the chamber cut, then neck-size a few cases (using a bushing-type sizer die) and fire-form them in the gun. Send these off to the die maker and have a set of full-length sizing dies, or whatever you like, made for the round, and you're off.
Did I miss anything?
If you aren't looking to design a new round, but just make a straight 204-223 WSM, you could just have your smith cut the chamber with no neck, then cut your neck and throat. You could buy a 223 die set off the shelf and a neck sizer and there you have it.