Drive a soft slug of lead through the barrel and measure the groove diameter (multiple times with a vernier mic). There's no point is loading any diameter bullet if it will not be at least .0005 larger than the groove. You are more likely going to run in to excessively large throats however which has no easy fix. Ideally, if the groove is A, the bullet should be A+ .001 and the throat should be as snug around this bullet as possible with a slight interference hurting nothing, about A+.0005. So it's Groove dia, then Bullet dia., then Throat dia.
Welcome to the world of cast bullet shooting.
The only other piece of advice I have is to make sure you are using/ buying the highest quality lube available. With less-than-desirable dimensions in one of my own guns, I worked up countless loads with variations in other variables (bullet hardness, shape, forcing cone dimensions, powder type and charge, and countless bullet diameters) with poor results (We are talking about both accuracy and freedom from leading) - all with Lee liquid Alox. When I went to a high quality, traditional lube, life got much better even with an oversized throat; not great, which I have achieved once in my life with a K-38 that has perfect cast bullet dimensions. So I know it's possible but not wiothout that dimensional relationship I described. Of course, gas checks will cure much.
