welcome MaxMatt!
Nice cars! How did you source an Eldo in Poland?
Although I'm sure the MB's could see an improvement in consumption, I'm not so sure about the Cad. If it's got the single exhaust, I'd do a true dual exhaust swap first. Then just make sure everything else is in good tune. Does it still have the original Q-jet carb? If not, try to get one as on the older GM cars it gives you the best possible FE for a carb system. If it's got one already, make sure it's in good shape. They do need rebuilds occasionally, if it needs reconditioning make sure it's done right by someone who knows them. A swap to an HEI type ignition versus a points distributor might help with efficiency slightly. Just a sidenote, the best I've ever seen an old Cad like that do is about 14 mpg, and that's highway mileage.
Once again, welcome and enjoy the stay!
Aaron