Jacketed bullets are very forgiving as to fit. An undersize or oversize bullet may work fine. I'd try at least two diameters, starting with .308.

With cast, use the largest diameter that will allow chambering a loaded cartridge without difficulty. This will likely provide best accuracy (providing alloy strength is compatible with pressure / velocity). Very slight engraving of the bullet nose or body, whichever happens first, is important with cast bullets in a rifle.

Slug the bore if you wish. Sometimes that's helpful; usually it's not necessary.