Highest ratio from the factory was a 4.10
Highest from the aftermarket was a 4.56
Posi yes, but I wouldn't put any money in that rear end.
When I was younger they were plentiful and cheap so I could blow up one a week and rebuild another out of the junkyard. I can not recommend that attitude today as they are not plentiful though they are still weak, and subject to failure if abused. Even a 283 can break one if you side stepped the clutch above eight grand. Only justification for a 283 is to shift it at ten grand like I used to. But then again 283's were plentiful back then too, and they used to fail at that RPM frequently.