Necking down for a wildcat

[Mikecr]

So you can't fire form, and standard bushing neck dies are partial neck sizers. I don't know if there is a true FL die for your wildcat.
Then you really need a form die here. Something to get you close enough to fire form in your new chamber (per your design).
Mine is an old Lee Six w/bushing set, but I don't know who offers form dies today..

I wouldn't worry about donuts with downsizing, but you do need to see what your down sized neck thickness is, and account for it.