Nope, the 351W came out in 69. In 68 you could have had a 302 , 390 , 427 or a 428
Your VIN will tell you which engine you had . Give us the VIN and I'm sure someong here can tell you what your original engine was..
I could be mistaken. However, I believe a Cougar only had to have the GT trim package to have a 390. I've seen quite a few '67/8 Cougar GTs with 390's that were not XR7's. Also I believe the '67/8 Mustang(uh-oh I commited a sin....I mentioned 'Mustang' ) had to have the GT trim package for a 390. I can look it up though, and double check. I do believe that a Cougar and Mustang only had to have the GT Trim package(and the 'Handling' Package) to have a FE.