I agree, one of the big Caddy motors, either the 500 or 472 would be a great choice. They make their power at low RPM so are perfect for moving a heavy car. Keeping matching numbers in a modified car is not real important value wise. Parts, including High Performance, are plentiful for the 425/472/500 motors. A great source of Cadillac info is available at Maximum Torque Specialties: www.500cid.com. They also have a forum with lots of knowledgable and helpful people

Good luck with your project and keep us posted. Some pics would be great.

Pat