What's up with the LA Dart engine?

No it didn't. The poly engine used the same bellhousing bolt pattern as the LA.

SS/A Darts and Cudas were not aluminum. Bodies in white were shipped to Hurst where the Hemi and a fiberglass front clip was fitted.

Some of the 64 and 65 SS packages had aluminum front fenders and hoods, but that was it until 76 with e Feather Duster and Dart Lite. But in all cases the unibody began as a production unit pulled from the assembly line. Many racers acid dipped the unibody to lighten it, which caused premature corrosion. This is why few original SS cars survive today that haven't been rebodied