About the lead bullets: it depends entirely on what velocity you are shooting them at. Assuming that you will be shooting commercially "hard cast" bullets, they will work without much fouling if you keep the velocity to about 800fps, or between about 1200fps and 1400fps. If the velocity is in between that window, or over 1400fps, you can expect fouling (depending on your gun).
If you cast your own bullets and cast them softer (like with Lyman #2 metal), you can shoot within the 900-1100fps window without leading.
If your gun was made during the last 30 years or so, and is of a decent manufacturer, I don't think you need to worry about bore size vis a vis bullet diameter. They are so uniform in modern guns and bullets that you just don't need to worry about it.