pamtnman,
You should be able to safely use any diameter bullet that will easily fit into the unsized neck of a case fired in that rifle. If the case can't expand to release the bullet, pressure will go up, but if the bullet is able to move before it enters the rifling, pressure won't go up significantly. This is true even for jacketed bullets and lead bullets are even less critical. I regularly shoot .321" bullete in a rifle with .318" groove diameter and .364" bullets in a 9.3x72R with .358" groove diameter. By the time a bullet has traveled it's length in the barrel, it has been sized to fit. I don't think you would be limited to the smallest dimension of the oval bore.
Mike