I have a '67 mustang coupe and am considering swapping the 289 it has in it for a 351C. I know that there is very little work involved in dropping in a windsor motor. But, i have heard that the cleveland is wider than the windsor. I was just wondering if it is significantly wider that is requires modifications to the shock towers, motor mounts, etc. Thanks a lot.
351C owners (in stock trim its undoubtedly the most awesome SBF ever produced), IMHO the 351W is the better choice. The 351C is, once again, an awesome engine, but in comparison to the 351W, it has suffered from little development of aftermarket parts since it hasn't been produced since, I believe, 1974. Just my $.02 though.
As rbohm said, the headers are the only real difference. There's room for either an FE or 385 series big block there, the Cleveland is a nice fit in 67-70's. As for parts being hard to find, that's B.S., pure and simple. The parts are out there, more and more everyday, you just have to open your eyes and look. 
Rbohm and I just said there are more parts available for a 351W, which is undisputable.