Be aware that part numbers and engineering numbers (the ones stamped on the spring themselves) are two different things and are different. A 69 Mercury Master Parts Catalog should list both numbers but not sure how much access there is for these catalogs for Cougars/Mercurys. For Fords they and later versions of the catalog are everywhere.
BTW you didn’t list if your car was a coupe of a convertible which often makes a difference in this wort of thing. Guess there are at least 4-6 different rear springs that were used that year on 351 Cougars based on the build sheets I pulled up