Here is the link to Lincoln & Mercury parts books online:
https://www.lincolnoldparts.com/parts-books
I looked up the heater core for a '57 & '58 Merc without air conditioning, part # shows
FEK-18476-A. With factory air -
FEK-18476-C.
But - double check to make sure it is for your application.
I didn't do a cross-match or check for updated or replaced part # but it should be a good starting point. Good luck