69 throttle cable (351C) length

could anyone tell me the correct cable length for this conversion or how to determine the correct length. Also the name of a supplier??? I know this is a lot to ask but I'm really in the dark on this subject. I think someone out there may have done this conversion before --any help would be appreciated!!! btw this is a standard '69 mustang with a 351C and a c4 tranny.