Think about it. Ford created the Modular line of motors in an effort to improve interchange across different product lines. Why? To save $$'s.
The long blocks are the SAME. What makes the performance different between a Mustang and a the Panther line are things that are basically bolted onto the motor. For example intake, fuel delivery, exhaust, and perhaps most importantly. PCM TUNE.
If you heard that the Town Car's motor is different, it might be helpful if you researched exactly which model years in question.
Clearly any Ford car that came with a DOHC motor is going to be different from the SOHC used in the Mustang.