For me it would have to be sized .440 or .445 Kelly
If your barrel slugs to .435, then from everything I've read, your optimum cast bullet should be a .437, or .002 over bore unless you have an oversize throat, in which case you need a bullet just a hair under throat diameter so that it doesn't have room to tip on firing and enter the rifling crooked.
Have you done a chamber cast or impression yet? If so, what diameter is your throat?
Ian