In Australia they did a lot of wildcats on the .303. From what I remember off the top of my head is that the actions were not all that strong for high pressure loadings, maxing out around 45K. They had some luck with .22-303's but it was the pressure limit I mentioned that stunted it's popularity. It was the .25-303 that was the most popular, and I recall this because I looked at a lot of wildcats when I was doing research for myself.
This doesn't help you with finding a gunsmith in the States but might get you going in a positive direction. If it were me and I was set on making a project out of what you have it would be a .25-303.
Why?
Because after you put your money and effort into building this rifle, if it wont shoot accuratly enough to be an acceptable 200 yard varmint/predator offering odds are that it will shoot well enough to handle medium game rifle to about that range.
There is enough bullet selection in .25 cal to cover both genre, and the nice thing is that if it works out you would end up with a truly versatile piece.
Good Luck,
Respectfully,
Blaze
