Have you tried AutoKrafters yet? They seem to have a decent stock of motor mounts...I'll dig out my catalog and take a peek, shouldn't be too hard. Escpecially since your motor is a 351C, not a swap-in motor or anything.
HI there hey the last time I looked for those mounts I couldn't find them either. I finally got a hold of the guy I bought the car from and he gave me the original 351C mounts. Now I say this because my motor is a 351W, so the mounts for the Windsor or the clevelend all worked fine. The only real difference in the block I believe is the deck height and the heads. I f you still have no luck try Kens Cougars, see links or maybe some of the other cougar vendors. Hope this helps..