An MSD won't work without completely changing over the entire fuel and intake system. Assuming this is a '79 with the 368 and EFI, there will be other issues to consider. If the car has electronic climate control, the HVAC system was also tied into the computer system and these controls may not work.

If this is a '76, this should still have a carb and standard type ignition (possibly GM HEI). Even if there is an EFI, the HVAC is a seperate system on the older models. I believe the only Caddy with EFI in '76 was the SeVille.