Magnum manifolds on a 66 dart

With manual steering, you stand a chance. Neither manifold choice will work with power steering. If you're using column linkage, probably not. You will need to cut back the outer column (almost to the firewall), probably eliminate the pot coupler in favor of a U joint, and possibly do a bit of clearance grinding on the manifold for column clearance. And most likely do some motor mount shimming to fine tune your clearance. It's tight, to the point that you may or may not need to do all of it- production tolerances are sketchy and what one person gets away with may not be enough for the next guy.
Even with the 340 manifolds you'd have to do most of that, anyway.