The 40 and 80 wire cables are interchangeable. You won't hurt anything by using one or the other. You may get better performance with the 80 pin cable, but data rates that high are typically only used by hard drives anyway.
Both 80 and 40 wire cables have 40 pins. If you do want a long 80 wire cable, you can find then easily in many computer supply stores and websites. (Antec makes a nice 26 inch version.)