An 8.1 would be sweet, but you most likely wouldnt be able to use the 465. Im fairly certain you would have to use the stick that the 8.1 comes with as that thing has wiring for days. And I know the tranny has electric components that the 8.1 would need for its EFI systems. Swapping to a carb would be a thought, but I have yet to see the 8.1 with a carb, as the heads are completely different than oval port 454 heads. The one I just finished would destroy a 6.2, or a 6.5 for that matter, all day on any surface pulling anything period. With the exception of gas mileage, but it does average 15 which isnt horrible.
The Cummins would be cool as well, way better power.
But re doing the 6.2 would probably be your cheapest option and easiest to do.