74 Duster (318 CI) to 440

No on any of the accessories swapping over.

Those are your best header options but the Hedman shortie headers may fit with a small starter. Some have used manifolds (search threads here).

750 vacuum secondary carb or any other 750 is fine.

If you ever put sticky tires on it that 8.25 rear will go on permanent vacation.

Hope you didn't over cam it,those pistons look pretty far in the hole