If you have access to a lathe then a 4-jaw chuck, centralised using a DTI on the existing bore, then bored out to 80mm. Creep up on the final cut as it's quite a deep part.

If you have access to a milling machine then a flycutter, or a proper boring head if it will cut a part that deep. I don't have one myself but imagine they are expensive.

Do all this before you make the slit etc.

If you have neither, then do you have the receipt! Would the supplier swap it for an 80mm version?