I'd either suggest a Ford 200 or if fuel mileage is really important, consider a 2.3L or 2.5L 4cyl out of a 90s era Ranger. I'd suggest the 5spd though if you want gas mileage, whether the four or six cylinder. Its been long time since I've even thought of a Ramber American let alone seen one, but this was a smallish compact car if I remember correctly, so fuel injected Ranger engine would easily handle things. And unlike finding a good condition 200, finding a good used Ranger 4cyl engine is easy and they usually very cheap. Fuel injected version, you would need all the computer harness stuff and the like. The 90s era Ranger engines were very long lasting with some regular maintenence.
If you have surplus money, you can get a brand spankin new 3.0L GM marine engine. Its the old pushrod carbureted 2.5 Iron Duke 4cyl with longer stroke and no pollution stuff. Made for use in boats, but shouldnt be problem using it in a pre pollution car. Some guy makes a kit to put them in ancient Landrovers where there isnt room for a six and parts for the OEM engine are getting expensive.