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Oops,
Sorry, just reread your post and noticed the frame is bent. That changes things.
While putting your old engine in will be a slam dunk, swapping out all the rest of the running gear will take some fab work, and again, it will never be as “right”, but that’s a personal decision.

As far as the transmission goes I have driven many of both types and would personally take a 420 over a 465 any day, but I know others will argue that point.

If the 221 is in good shape I would just leave it. Bearings and gaskets are not that big a deal if needed and the divorced setup is easier to work on because you don’t have to drop the transmission. If the gears are toast then parts can be difficult. It also runs louder. Driveline angles are better though, because the outputs are lower, that’s why earlier trucks ride higher. Unless you’re out doing stupid stuff either case with last just fine

Hope this helps