I'm guessing that the bore is probably 79mm which is the generally accepted max for these engines & it's 3mm over the original size.

I think you should hang around your dad as much as possible as he obviously has much to teach. He sounds like a good man, so you had better keep this one.

If you have a ute, then the diff will probably be a 4.11 to 1 ratio. By switching to a 3.9 to 1 ratio you will lose a tiny bit of acceleration but gain in driveability, particularly on the highway. It's all good.