In my days I have swapped many c body 68-70 440's into 66-70 b bodies. With all new mounts I have never had a problem, actually the c body pan in most cases has a extra notch for the idler arm. The problem I have seen is the body of the pan is a little deeper and will allow the center link to rub the pan.
What year C body pan do you have? If it's a 68-70 with new or solid mount I would say you are good to go.